Matt,Another great question! I like the fact that you never just except the answer but want to know why. I also wanted to wait to see if anyone else answered your post because it seems I am always answering them.
I don't know an aswer directly but I think it would be a biomechanical reason. I would think if you are not finishing your pull (i.e. you don't get full trunk extension) the tendenancy would be to jump forward to get under the bar. If you over pull (full trunk extension plus a little), the bar would travel backwards and you would jump a little backwards to get under the bar.
So, if this is true, you would not want to jump forwards because you are not reaching full extension. You would want to jump backwards because over pulling a little will usually result in a good lift.
See you in July at the Keystone State Games!