Let's see if I can be really confusing as I try to explain this without a diagram...

It is the horizontal distance to your target that determines the amount of arrow drop, not the line-of-sight distance.

For example: A deer is standing 10 yards away from the base of the tree you are in. The actual distance from you up high in the tree stand to the deer on the ground might be 15 yards. If you hold on the deer as if he were 15 yards away, then your arrow would strike high because he is only 10 horizontal yards away.

To be accurate when shooting uphill, downhill or from treestand, you must know the horizontal distance that the arrow will travel. Many archers, myself included, tend to shoot high when aiming uphill or downhill because we don't accurately estimate horizontal yardage to the target which will always be less than the line-of-sight yardage. In other words, we tend to aim too high on shots that are not made over level ground.

I have confused myself so I will stop.