Iron Golems Spawning on Slabs

Although its quite rare I noticed in one of my iron farms that I recently built on a mc bedrock realm that iron golems are able to spawn on half slabs. I dont know the reason why but they tend to spawn above the villagers.The video I have attached was compressed in order to fit the 10MB requiremnt, so if the full video of my afk session in better quality is needed I can also provide that.