The way GW interprets it I think is that skeletons have some small remnant of their souls remaining and so they know roughly how to fight without direction. Zombie on the other hand only retain the knowledge of how to feed and as such are easier to outmanouvre.
In response to that, the idea that skeletons are better than zombies because of what they were able to retain from their prior lives doesn't tally. If so, does that mean that those able to retain the knowledge of how to fight instantly lose their flesh and become skelebobs? And if Zombies are zombies because they only remember how to feed, what happens when the flesh, well, decays completly? Do they just disappear? I'm not knocking your view, I just can't get my head around it.
I'd suggest that possibly having the flesh makes zombies more cumbersome for a
necro to command possibly? The lack of flesh enabling the
necro to be much more agile, well, as agile as you can be being a corpse. Just another possible take along the lines with Santawraith (although, having never played D&D, it is all a bit alien to me =P)
OR *drum roll* the brain, as Ophidicus mentioned, might be the key? As Ophidicus mentioned, they could be still thinking? Now I understand the idea of conflicting life and undeath minds, but I also wonder, do they have more semblance of will than a skeleton (in the form of their newly dead cranial matter) with which they resist the
necro?
I've seen skeletons being more powerful than zombies in a few places:
Warlord's Battlecry III, where zombies were low healthed, slow, builders, whilst skeletons were the only real infantry (but you upgraded them through wight, to death knight or liche etc). It kind of made sense.
Oblivion where it kind of had zombies being able to take much more damage, but being slowler and clumbsy (and gave out less damage as unarmed) whilst skeletons, although weaker (discounting shields etc) caused more damage due to being armed, albeit poorly.
I think that had it more in line with how I'd perceive it-Zombies harder to kill, striking last but in greater number. Skeletons hurt more and hit back more individually but are potentially a lot weaker.
Umbreus