If you need to aim higher for uphill shots the only reason I can think of is that you are shortening your draw length by not bending at the waist. There are only two limiting parameters to arrow trajectory- gravity and wind resistance. Gravity doesn't care whether you are shooting up hill or down hill or on the flat, for our purposes it effects an arrow the same for the same "horizontal" distance.
Try this, have your target up in a tree, hill, etc. Draw on the horizontal and then bend backwards in order to shoot. My bet is that you will struggle to keep your draw length the same. Either that or put a cloths pin on your arrow and actually measure your draw length in both scenerios.