I might add, it also depends on how far away your target is.
The bullet does not travel in a straight line--it arcs--whereas your line of sight is (ideally) straight. Assuming the weapon is sighted in, the arc of the bullets path meets the line of sight at two points (zeros). At these zeros, the point of aim will match the point of impact when the weapon is fired with perfect accuracy (as from a mechanical rest). Nearer or farther than these points, the point of impact will be higher or lower than the point of aim. The sighting will also vary with ammunition (i.e. lighter, faster bullets shoot flatter).
For example--the way my M&P 40 is sighted (and the way I shoot when I am having a good day), the PoA = PoI @ 7 yards. PoI is somewhat lower if I am shooting darn close (e.g. 3 yards), somewhat higher at 15-25 yards, back to zero at maybe 40 yards, and naturally gets much lower at extreme ranges (for a pistol) like 100 yards. Another way to put it is this: if I hold my aim at the center of a bullseye at 7 yards, I will hit the center, but to hit the center at 25 yards I hold at 6 o'clock.
Your experience may vary depending on how your sights are installed/adjusted, but where the bullet impacts relative to where you hold your sights will vary with the range at which you are shooting and the type of ammunition you are using.