Sometimes, the walls react as if there was a block above them

What I expected to happen was...:
the middle bottom wall to be flush with the others walls

What actually happened was...:
the middle bottom wall reacted as if a block where above

Steps to Reproduce:
1.Place three walls in east-west direction
2.Place three more walls above th three first (make sure the last one placed is the middle one)
3.If done correctly th wall in the middle bottom should react weirdly