Both with and without Optifine in all versions of minecraft, including snapshots.
The legs of all mobs that use part of the player's entity model (including zombies, pigmen, zombie pigmen, and zombie villagers all have their legs offset backwards by a few pixels from the rest of the body. It also works for players. I noticed this when looking at someone who was AFK on my build server so I took screenshots of some of the mobs that this affects. It really seems quite pointless and I hope it is fixed because it bothers me.