Skeletons unable to damage the player when in a 2x1x2 area

Was playing survival when a skeleton came into a 2x1x2 cave area I was in. He started shooting, but every arrow went through me and into the wall behind me. But when I was not against the wall, he could damage me from any range.

EDIT: I should also mention that if a the skeleton was more than one block away from me, he could damage me.