I had that thought too. Have the numbers be the real enemy, not variety. --- Dragon's Dogma, Red Dead Redemption, Grand Theft Auto IV, Elder Scrolls: Oblivion, and Minecraft...5 of my favorite games ever.
I actually would not be surprised if they had zombies with different mechanics starting to pop up in later levels, purely for the sake of gameplay. It seems like most games do this regardless of how true they intend to stay to the source material. Personally, I hope that they don't do this, a game with nothing but 'dumb' zombies would actually be a nice change of pace to me.