Why feudal Europe (and, well, anywhere else that practiced feudalism in its strict definition) had such confusing power structure is fun.
There's this thing called the 'feudal pyramid' that ostensibly simplifies the feudal system's ranks and structure. That's helpful. You know, helpful if not for being a crock of utter bullshit. Feudalism can largely be boiled down to a simple exchange; A grants a tract of land to Person B. B, the vassal of A, the lord and liege, is obligated to provide taxes and military aide drawn from said land. The inhabitants of that land, C, the serfs or peasants (usually, sometimes they were outright slaves, others were far more free, we'll get there), service the territory and provide soldiers and taxes to B, their own lord, so B fulfills their obligations to A, the superior lord, in exchange for protection. And yes, lots of lords; lord really does just mean a man of superior station. It's why in English in Christianity one of the common terms for God is 'the Lord'; as in God is superior to all others, so is everyone's Lord.
Anyway, what this means there are tons of local variations, because that is stupidly simplistic. And stupidly simplistic also means stupidly common problems. They come from things like that a vassal may have multiple lieges. If you inherited a chunk of land under the rule of another lord from your current lord, were given one for mercenary service under a third one, and got given another fief as a wedding gift to a fourth liege, technically you had four different bosses (those used because I was thinking of a specific French knight that happened to; effectively he served the king of Aragon, two French dukes who served the French king, and the English-serving duke of Aquitane in France). And what if one went to war with another one? You were legally obligated to support both, but you logically couldn't, so you had to break your oaths. Survive war, the person who you broke the oath with, they could just seize the land you'd been given (naturally, why wouldn't they?) but they could also do stuff like sue you for damages, or try to get you excommunicated (since oaths were sworn in a religious context). It was like a matryoshka doll of headaches.
Other fun things included charter towns. As economies grew and stabilized, cities would sometimes position themselves to be quasi-independent entities in kingdoms by being granted a royal or imperial charter. The idea is that if they tended to their own affairs they'd better be able to enhance their economy. That took them out of the broader feudal milieu though, and tended to infuriate the local aristocracy who could either no longer tax them at all, or could only a tiny amount, since the charter typically had them paying somebody higher up than any local nobles.
Oh, or feudal territories without anyone assigned to actually administer them at all, and the locals, while ostensibly subjects of the kingdom or duchy or whatever, really just had to look after themselves. The 'anarcho-syndicalist commune' in Monty Python and the Holy Grail? That seriously, in itself, was not much of a joke. That was close to a very real legal structure peasants sometimes put together when they were essentially forgotten about. As in, you could totally slip the mind of your superiors for decades, wonder when the hell your new liege is coming, and go, "You know, we really need to organize a militia and a basic court system because shit is dangerous".
Speaking of, you had the weird position of common people in general. Peasants weren't just peasants everywhere. For example, St. Joan of Arc's father, technically a peasant. Jacquot (Jacques) D'Arc was, however, treated more like a nobleman than a lot of French nobles. In Domrémy he was so well respected he was appointed as 'dean', which gave him governmental, diplomatic, and military authority like he was a knight and ambassador, but technically he was still just a peasant who sometimes got to boss around men who were, on paper, his feudal superiors. Compare that to societies, still feudal, where a man like him would've been a slave in everything but name. Some places, a serf could be killed by a superior for no real reason at all; they were classed as property to the extent it was seen as little different than intentionally breaking a plate or chair you owned.
Or fun, cases where the society wasn't caste, but class-based, and feudal ranks shifted almost entirely based on a combination of personal wealth, reputation, and popularity, and land was maintained by military preparedness; if you were strong enough to keep and protect it, you had a legal right to it, essentially. That was how a ton of Slavic countries worked, Gaelic Ireland and Scotland, Scandinavia for a long time, and various Asian polities on the Chinese hinterlands, despite often all engaging in land divisions that followed the feudal model, albeit with elements determining who could actually give or be given land (that is, respect to the tribe or clan or insert-familal-unit-of-choice who owned the area traditionally) barring a serious war or other major event that dislodged one from the area.
The Ui Liathan of Ireland were arguing off and on into the 14th century, at least, they technically owned a chunk of central Wales, with whoever technically controlled it at the time, because a bunch of them had been given it as mercenaries by a local British chieftain around 7 centuries earlier (a common occurence, after the withdrawl of Rome, British communities had little protection, often from the Irish themselves, hence the invitation of Germans as well), and the descendants of those people were really just more Welsh people by that time, their ancestors having long married into the local population over many generations to the point of their Irish ancestry being irrelevant. You know, except to the ones who were like, "Nope, legally that should still be ours" based on a land grant so old that everyone involved's great-great-great-great (this goes on) grandchildren were dead, and the legal system it was done under no longer even existed.
And that's another fun one; shifting legal systems. Because the way contracts were made and functioned changed over time, sometimes pretty rapidly, it was hard to translate earlier legally binding terms into new ones. In the Ui Liathan case, that was reliant on a special agreement between one of the tribe's chiefs and a Welsh equivalent that was probably mostly or entirely oral and taken on honor. Centuries on, agreements like that required physical documentation to prove it so you couldn't just claim "My so many greats ancestor made an agreement with yours".
Then you had conquering lands that had an entirely different legal structure than the new overall ruler. That could get baffling real quick since the exact same things, on paper, often had entirely different rules. Like, local 'knights' or their equivalent may be entirely an inherited position, or elected, or by appointment, or maybe an admixture of different methods. This, again, reinforced the need for very, very clear documentation, not just to be clear on what was once agreed on, so that the various legal minds of the time could try and translate one legal system into another with minimal amounts of breaking the entire local social order and causing total anarchy (and if you don't know, those crack feudal lawyers failed at that a lot of times).
Of course, that documentation also had problems, since forging geneologies to prove rightful inheritance, or completely fictional contracts that would've been totally impossible at the time they were said to have been made were both extraordinarily common. So, so many feudal documents for land claims actually claim to be from time periods where the land in question didn't even necessarily exist, like claiming lands in Normandy before Normandy existed and was still called Neustria. Or claiming a person there was no proof of existing as a biological ancestor giving numerous legal land claims or claims to certain objects (King Arthur, Ragnar Lodbrok, kind of looking in your guys' direction).
Incidentally, don't think you ever see that in fantasy. Just somebody calling out a bullshit claim due to their prodigious history education proving a claim can't be legitimate since a place didn't exist yet, or a person was determined to have almost certainly not been real (or at least, a single real person; lots of legendary figures are likely composites).
Kings, kind of weak for most of the feudal era. Kings generally did not rule by fiat or bureaucratic commands. They often tried to, but even in highly organized places, even emperors were routinely told to piss off unless they could offer something of value. Power was much more often a series of neverending negotiations and renegotiations between individual nobles, towns, religious authorities, and populist movements demanding legal changes (which happened way more than most people think), who all had their own internalized support structures, interwoven alliances with others, and often at least some degree of force of arms to back it up.
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