I agree with the original post though wording Im not sure about. Using a 20 yard pin, if you have an upward arc, with a horizontal shot you are shooting against gravity. This in itself will slow the arrow. Shooting downward you do not have an upward arc that is moving against to the gravity field. You are closer to parallel. The closer to parallel of gravitational pull you are, the less resistance to your arrow and the farther it can go. It has nothing to do with how long it takes to drop, it has to do with the forward momentum and how far it will travel before its time is up.
Stryker777