It's hard to say without all the info. As someone suggested, it could be a kenetic energy problem. BUT, since you didn't see the arrow fly all the way and you hit so far from where you were aiming, it's possible you deflected off something on the way and had a slapping hit All sorts of things could happen. Another, at the moment of truth people sometimes don't realize they've changed their form. Grab the bow like it's a life and death matter and you change the shot. Grip pressure changes are probably the number one reason for errant shots in the woods.