Think about it in the extreme sense. Your arrow will drop about 6" between 20 and 30 yards. If you had the worlds longest sight bar (20 yards long), your pins wold be about 6" apart. As you bring the sight in closer to you, you are essentially closer to the adjacent angle of the triangle so your pins get closer together. In the most extreme sense, if you pins were directly in front of your eye, at 20 and 30 yards, they would be touching.