My guess would be that without the arrow, he is able to hold the string much more securely and rip it back, allowing him to pull more weight than he is actually capable of shooting. When he holds with an arrow, he has considerably less strength at the ends of his fingers, and so can't actually shoot the weight he can pull when able to grab the string with a full grip. I can pull 20 more pounds than I shoot If I don't have to actually shoot it. That's just my guess, I could be wrong. If you have a very light bow, try it and see if he is suddenly able to reach anchor; you'll know then. Paul.