This may be a simple question to some of you more knowledgeable guys, but I suck at physics and need an explanation. I recently purchased a flat top upper for my AR because I didn't like having the scope on the carry handle 7 inches above the barrel. I installed the scope on the new upper and fired a couple shots at 25 yards to make sure I was on paper. I was only an inch or so low so I corrected the point of impact and moved the target out to 50 yards. At 50 yards I was grouping about 5 inches high. I lowered it and moved it to 75 yards. Now it's three inches high. Another correction and moved it to 100 only to be 2 inches high. Another adjustment put it steadily in the center. I ran out of ammo and didn't get to see what happens if I move it back to 50 yards. It groups great at a set distance Any ideas what's going on?