Need some help on why you can use finger bones on body meshes.

[wapeddell]

So basically I'm trying to make my nif consist of one mesh but for some reason whenever I try to rig fingers to the body they get distorted. In other words, when I apply weight to them they go all the way down to the feet same goes for the head. I don't understand the bones in Fallout 4. I've been able to do this in Skyrim, New Vegas, and several other games. I know that other models like the Supermutant have a full body and I'm able to rig to it. However, in the game it doesn't work right but they are the same exact bones as a human just in a different pose.