Passive Mobs spawning above chunk maximum

I have been building in creative and noticed that the passive spawns keep increasing. I turned on chunk borders, put a fence around it and killed all passive mobs in the chunk and came up with the following. 12 cows, 12 pigs, 6 chickens and 9 sheep (all white)

I noticed this in pre5, but passed it off to being a bug that someone was bound to report. After killing all the passives in that chunk they have respawned in the chunk next to the previous at the same rates. 

I checked this in a new world in survival and it is still just as bad. I also noticed a "naturally" spawned pink sheep!?