Zombies not spawning in caves

Zombies are no longer spawning in caves. I did a test on spawning with platforms every 10 levels and commands that place a colored wool block where mobs spawn. The wool for zombies were only found on the top platform which would have been surface, nothing in between platforms.

 

In the attached screen shots the green wool blocks are zombies spawns, lime are creepers, black spiders, purple enderman, white skeleton and red witches.