Skeletons shooting through upside down outward stairs.

I have an xp farm with skeletons and I'm using an upside down stairs block to filter the XP from. The block is facing outward towards me and away from the skeletons but I get shot at every once in a while while standing there. I believe it's going through the block and to me, I have a 2x3 room with torches where I afk.