overworld mobs spawn in the nether such as a normal zombie or enderman

going through the nether and have seen normal zombies, and enderman spawn 

What I expected to happen was...:
overworld mobs shouldnt be able to spawn in nether only come through nether portals or through commands and eggs

What actually happened was...:
the mobs spawned infront of me

Steps to Reproduce:
1. run around in an area killing pigmen and eventually one will spawn
2. ...
3. ...