Board Thread:Questions and Answers/@comment-71.81.112.73-20130405174641/@comment-75.109.154.42-20130413040002

It's the same reason there is weird lighting glitches with half slabs. I think the game recognizes the slab (or whatever block you're using) as a whole block as far as lighting is conserned. Its the same reason you can look through stairs and see whats on the other side without light coming through