Iron Golem Spawns in the Wall

What I expected to happen was...:
Iron Golem would NOT spawn on the block which has glass at two block above

What actually happened was...:

Iron Golem spawned in bed, stone-brick, glass, and chest

Steps to Reproduce:
1. Lay beds in the hall made of stone-bricks and glasses. In the picture are probably 116 beds.
2. Let Villagers breed as many as possible.
3. Wait until Golems spawn automatically. Some of them spawn in wall, beds, or chests.