It seems after killing a compressed mob and then all the mobs that spawn after it they are still being counted as being there by vanilla code causing the Ender IO powered spawner to think there are too many mobs in an area.

https://www.reddit.com/r/feedthebeast/comments/5lx9n9/sky_factory_3_powered_spawner_bug/

http://i.imgur.com/tZaI4iB.png

Is that what it is! oh woww!

I think this may be fixed by v2.0.76.