Zombies are dumb and slow because of all the rotting flesh. They're too fresh. When raised, they are still clinging to the remnants of their past lives, and are therefore confused, slow, and for the most part brainless. Their muscles are rotted so they hinder movement rather than help them (the movement mechanics are magical now, of course) so that explains the shambling.

Skeletons, on the other hand, have been dead for awhile. When raised, they're not so intent on clinging to the past. Plus, the lack of rotting flesh that slows down zombies helps them be nimble and quick--I mean, imagine how fast YOU could run if you only had to haul around your skeleton instead of pounds and pounds of flesh and blood!

Since they no longer spend most of their time wondering why their dead or any of the countless other silly things zombies wonder about, they've got more time to think about how to skewer unwary travellers in the night.

And they cackle the same way all undead creatures do anything. Magic.

Of course, I made all of this up. But that doesn't mean it's not TRUE.

-- Dave