ORIGINAL: Wingbone
It could be your form. Make sure you are bending at the waste when shooting down hill. If you are merely lowering your bow arm, it changes your form and the way you are looking through your sights. However, if you are shooting at longer ranges (30 yd+.) at extreme angles your arrowwill actually have less drop than shooting horizontal, but at closer ranges most bows are so fast you won't see a noticable difference.
Well said, Wingbone. I hope we don't re-hash the angle/gravity/acceleration thing again. The search function should yield lots of debate on this subject.