I also have another reason that makes sense. If you look at a deer lets say it is 18 inches from the top of the deer to the bottom. When you are above the deer you now can see the top of the deer, if you look at this distance as a whole it could be longer lets say 24 inches. So if you aim from ground level at this deer right in the middle you will hit 9 inches up. If you aim from the tree when it looks like 24 inches in the middle that will be 12 inches up, however the deer is really only 18 inches so you would have to shoot 3 inches low to compensate for this. I saw this on the outdoor channel all about shot placement. Very helpful.