| Ravingdork |
Variant Zombie require specific additional spells:
"Most zombies are created using animate dead. Such zombies are always of the standard type, unless the creator also casts haste or remove paralysis to create fast zombies, or contagion to create plague zombies."
Skeletons on the other hand, take up twice the effective hit dice when created (but not for the purposes of how many you can control:
"Numerous variant skeletons exist, such as those whose bones burn with an unending fire and those who drip with gore and reassemble themselves over time. Both of these variant skeletons can be created using animate dead, but they count as twice their normal number of Hit Dice per casting. Once controlled, they count normally against the controller's limit."
Variant skeletons, unlike variant zombies, do not require additional spells. Zombies, on the other hand, do not effectively double their hit dice when being created.
These rules can be found under the zombie and skeleton monster entries, respectively.
I don't know why the use completely different mechanics for the each type of undead.