I thought I explained this adequately, but apparently not.
The sight picture in the pic is correct FOR 25 YARDS!!!
If you are CLOSER than 25 yards you will have to aim higher to compensate for the barrel being below the sights. Thus, needing to completely cover the intended target (the amount you need to cover it will vary with distance) at distances closer than 25 yards. This phenomenon occurs because the bullet travels in an arc after leaving the barrel. However, at pistol shooting distances the bullet hardly has time to rise before striking the target, thus requiring a lower POA when shooting up close.
If you really want to experience this phenomenon, try shooting a rifle sighted in at 100+ yards at less than 25 yards. It will take some practice to hit on target, but it will clearly demonstrate why you need to aim higher at close distances.