mobs still spawn when gamerule is off

mobs keep spawning even when the gamerule is off.

What I expected to happen was...:
that turning off mob spawning before making the world will make no animals be all over terrain and to not naturally spawn in general.

What actually happened was...:
animals kept spawning all over terrain even when there were no nearby structures.

Steps to Reproduce:
1. create a new world
2. go to "game rules"
3. go down ultil you see mob spawning setting and turn it off

4. load up the world and look for mobs on terrain.