Quickdtoo is correct. If you think about it, bore sighting is a straigh-line plane. However, if you were to drop a bullet from your hand at muzzle height, and to fire one exactly parrallel to the ground at the same time, they'd hit the ground at exactly the same time. It still falls. So even hitting POA at 100 yds. requires aiming a little higher than bore-sighted POA in order to get the trajectory right.