The way a person falls, and the way the arrow went into the body can tell you almost exactly the direction the arrow came from. You can do it with bullets too.
I have people looking straight at me from 2 feet away while I'm in sneak and they don't notice, but when I fire an arrow, everyone immediately knows exactly where I am based on the direction my arrow was buried in the guy?
@ other guy Some realism in a game is important, based on your logic, I should be able to enchant a knife so that I can throw it through a wall and when it hits someone it will explode. Because, well, its a game, realism doesn't matter at all.