Hands are weird. They're the only things I know of that, when made into a different position, will be referred to as something else entirely.
When you clench your hand, it's no longer called a hand. It's called a fist. It's technically a hand, but you don't look at a fist and refer to it as a hand. Or even think of it as a hand. You think of it as a fist. A means of causing harm to something else; a collection of flesh and bone balled up to utilize force.
Maybe it's because of the intentions of the two that make them seem so separate. An open hand is used to fix things. To operate. To create. To help others. To type. To feed. To play. To hold. To touch. To do.
A fist is used to... punch things. To cause harm.
Maybe the two names are meant to distinguish between a human/animal mode when referring to people. When you clench your hand and it becomes a fist, there aren't a lot of complicated thoughts running through your head. It's a moment of tension and intention to hurt something for whatever reason. Whether or not it's for self defense, at the end of the day, that's exactly what it is. That's all it is.
I think that's the reason we have different words for the same thing, in this case. The intent changes with the position, and the difference is pretty noticable.