We have a lot of enemies. I think we maybe beat the total number of Diablo II monsters with the first two acts alone. (not counting bosses or any texture variants)
We generally have a number of texture variants for each enemy, mostly so they better match the feeling of the area where they're placed. Even within the same act an enemy may use a different texture so it matches the theme of the location. It also shows progression and helps the player recognize that the creature they're fighting now isn't the same as the one they fought before; sometimes because they're just tougher, and sometimes because their actual attack and damage types are different.
It can and does happen, as well as actual geometrical differences. It's more of an issue of time than anything. If we have the time to put in a bunch of variants to remove some of the visual repetition then we will, but it's a wish list item.