There is no reason other than the bow being slightly out of tune for the BHs to hit a different POI than the FT. Unless the BHs are heavier than the FTs. Make sure both FTs and BHs are the same weight. Sight in with the FTs and tune the rest until the BHs hit the same POI.
Shooting from an elevated postion will not change the POI as long as you maintain the same "triangle points" of anchor, sight and rest(grip). To do this you must bend at the waist and not stand vertical and lower the bow.