What everything does changing NPC weight do?

I always thought it's just a linear interpolation between vertex data of *body_0.nif and *body_1.nif, but there seems to be more. Because there are visible differences when I use identical nif for _0 and _1.

F.i. when I use just a static reference pose as a mt_idle anim, and I change my weight, my upper arms spread out slightly depending on the weight. How? Does changing the weight actually changes the Havok skeleton pose?

Also the neck seam. Why does neck seam actually change with weight, when neck edge vertex position are the same (provided I use same _0 and _1 body nifs)? *head.nif is just one, so it shouldn't influence anything weight related. Same goes for hair spearing slightly with higher weight.

Clearly there's more to NPC weight change then one morph across all vertices of body nif pair. Does anyone know more? What should I do if I theoretically want to make my character mesh look exactly the same regardless of weight?