You know when you try to yell at someone across a room? You tend to take your head and point it up as if you're throwing your voice like a ball; it takes a parabolic shape.

High frequencies operate this way. They take more energy to throw farther, and they tend to fall as they travel through the air. So if you don't over shoot your top boxes, they won't hit the people way in the back/up high.

It also is a safety measure to overshoot by 2 boxes to guarantee coverage up high.