Skeletons shoot themselves whilst using NoGravity-tag

Whilst using the NoGravity tag, skeletons occasionally shoot themselves.

What I expected to happen was...:
The skeletons just shoot at me and take no damage by that.

What actually happened was...:
The skeletons take damage themselves whilst trying to shoot me. Interesting here is the fact that they can damage themselves AND the player with the same arrow and shot.

Steps to Reproduce:
1. Set up a repeating commandblock with the command "entitydata @e

{NoGravity:1b}

"
2. Activate it
3. Spawn in some skeletons
4. Switch into survival mode
5. Notice, how they sometimes shoot themselves.