He doesn't wait for the arrow to be fully drawn back, thus it has almost no power and arcs downward rather rapidly.
Thanks! That makes a lot sense. In that case this is actually an awesome close range arrow tactic ^.^
Edit: I've never actually just tried tapping the attack button for archery before, but then again I don't use archery that much even on my stealth character anyways 0_o this looks cool though, makes archery more viable in melee