dreadful dirt spawning problems

[Quantumrage10]

playing on the atm9 TTS modpack i have been doing a bit of testing, trying to create mob specific farms, creeper ,spider etc. and I'm getting some super weird spawns happening slimes need 2.1 blocks of Hight, however I'm getting large slimes spawning in gaps as small as 1.5 the large slimes are getting stuck in the roof above them, other mobs such as enderman, zombies, skeletons seem to be working fine, and respecting their relative spawning conditions spiders also, cannot spawn in a 1 block high gap for some reason