That's exactly why I shoot three under. It is all about the sight picture. I don't use my arrow tip as a sight or try to put my point on anything as an aiming point. When I use 3 under though I can see my arrow tip on my peripheral vision. With split that point is somewhere on the ground half way to the target. With 3 under that tip is a lot closer to the target it'self, usually just under it. When my arrow tip is closer to the aiming point, it is a lot easier to align it left and right or guess the right hold over or under.

I don't consciously place the tip anywhere or even think about it at the shot but it just makes a repeatable sight picture easier to maintain.