Mobs spawn on new chunks even when doMobSpawn is set to false

Wanted to make an adventure map so wrote '/gamerule doMobSpawning false' to prevent animals,, and monsters from appearing, but after typing 'kill @e[type=!Player] and when travelling through map i saw some sheeps, pigs and cows show. So i repeated the command. After next 2, or 3 chunks I can see them presenting again! I think that shouldn't happen and didn't see another report about that, so I am writing this by myself.