When new chunks are created (flying through a new world), passive mobs are still spawned on new chunks if /gamerule doMobSpawning is set to false. This happens on default worlds but not in superflat worlds.

Steps to reproduce:

1. Create new world, creative, default settings
2. set /gamerule doMobSpawning false
3. type /kill @e[type=!Player] a few times to get rid of all entities
4. Fly in a straight direction
5. observe the entity counter in F3 and the passive mobs on the ground such as Cows, Sheep and Rabbits being spawned