Why do the animatronic's look different, near the end of the twisted ones funtime freddy gets shot 3 or 4 times and he breaks his jaw and half of his face plate is gone along with him switching between twisted and featureless. The wolf and Bonnie get set on fire were they have melted plastic over them along with some skin like thing and later get ripped apart. And there featureless state are more like skin and plastic then the metal look they have. And some more details I can't say
These are addressed in the game.
As for the slates, true, they are described as as a soft plastic that is translucent (I imagine something like transparent silicone molds as I think soft robots use that material). But drawing that kind of material in pixel art would take way to long for the amount of sprites this game requires, not to mention if I drew the slates translucent I would have to draw the chords underneath the slates as well, and that is just way too much detail for this. If I were to draw ONE picture of the mannequin animatronics instead of making an actual game, that picture would definitely have the effort of accurately drawing the material and all the chords underneath. But for this game, giving the solid slates a shine like they have is much more efficient for drawing a million times with sprites and their frames of animation.