Iron golem spawning ignores half slabs

I keep finding iron golems spawn on half slabs (block heights of y=#.5) but standing on the solid block below them (y=#.0). Also, I find them spawning in roofed areas with a height of 2.5 blocks. As I have read, they are 2.7 blocks tall. Their head is clearly in the roof. I'll take some screenshots if you need them.