I've wondered why headless zombies are tougher than regular zombies. Maybe it's the removal of the vestigal body part, lowering its center of gravity, therefore making it more balanced.
Muthsera,
I would proffer the most likely explanation first, which is simply "gameplay mechanics". However, that answer does not satisfy our desire for a more lore-friendly explanation, does it?
With that being said, I will extrapolate our current understanding into a hypothesis that is hopefully more to your liking.
The question presented is
"why are headless zombies more formidable opponents than their nondecapitated counterparts?"Here are some possible solutions:
1. The reason that the zombies are headless is due to their being violent criminals in life, meaning that they were executed for their crimes. The evil in their hearts make them prime candidates for the sinister magicka of necromancy, and due to their chosen lifestyle, were probably more physically potent than the average human anyway. This answer establishes a physical (versus magical) reason for their increased strength, etc.
2. The more "body" a zombie loses is the closer its spirit comes to being "freed" into pure spirit form, i.e., ghost, wraith, etc. This explanation is likely incorrect.
3. The loss of the head means that any vestiges of prior memories, pain, etc. would be fully removed, meaning that the PC is fighting necromantic magic itself rather than the husk-mind of the poor [censored] unlucky enough to get raised from the dead.
I am sure that we could come up with other possibilities, and the above are only possible solutions, not any attempt on my part to definitively answer your query.
My the Light of the Nine Bless and Keep you, for I remain...
___The Word Merchant of Julianos