Agreed. A down slope shot travels a modicum faster than a level shot because of the effect of gra\vity. Thus requiring less elevation compensation. I think it's called...angular momentum. Unless, of course, I'm wrong.
It is the vector. The direction of gravity is partially in the same direction as the bullet’s velocity. So it counters air resistance a bit and the bullet goes faster than it would if you shot horizontal. Shooting ‘up’ is the opposite. It is about the vector at which the gravitational force is offset from the direction of flight when you are shooting horizontal. So it deflects more.