Iron Golems Spawning Underground

i have an iron golem / villager farm, i wanted to make the inside of it golem proof as i wanted the golems to spawn outside, so i half slabbed most of the area. Now i get a few instances where the golems are spawning underghround. This was also happening before i slabbed the area. i have dug up all the floor and there are definately no gaps anywhere, the floor is solid for at least 5-6 down. Can this be looked into please ?

i also have a suspicion that the golems are spawning on the half slabs which i thought couldnt happen ? could that also be looked into ?

also wanted to point out that outside the farm are solid blocks on teh same level as the farm, which is really where i wanted the golems to spawn.