Every bow/arrow/shooter combination has its own unique point on, i.e. a certain distance where you can put the point of the arrow on the spot you want to hit. The problem is, if you don’t happen to be shooting at that distance, then you have to put the point somewhere over or under the spot (the gap), or ignore the point entirely if you want to shoot instinctively. The farther the arrow point is over or under the spot you want to hit, the less useful it is as an aiming tool.
The usual dilemma for a trad shooter is that he wants an aiming tool, but his point on is 40-50 yards, and what he wants to hit is 20-30 yards. So he has to hold the arrow point several feet below the spot he wants to hit, if measured at the target, or several inches, if measured at the bow. How do you reduce that gap? The gap is a result of the distance between the eye and the nock of the arrow, the length of the arrow, and the arrow speed.
To reduce the distance between the eye and the arrow, either anchor higher, tilt the head down, or hold the string somewhere below the arrow nock. Holding the string below the arrow nock is usually accomplished by string walking or using a fixed crawl. There is a good discussion of string walking by Ty Pelfry in Vol 2 of Masters of the Bare Bow, and of the fixed crawl by Jimmy Blackmon, which you can find on the Internet. The problem with any of these methods is that they are harder than simply shooting from a natural anchor with your head upright, so require some effort to master.
You can also reduce the gap by using a longer arrow, but there are obviously limits to how far you can go in this direction. Using a longer arrow means retuning, if your regular arrow is already tuned to your bow.
You can also reduce the gap by decreasing arrow speed, by using a heavier arrow or a lighter bow. Using a heavier arrow might be advantageous for hunting, and using a lighter bow might be advantageous if you are overbowed. Otherwise, this seems to me to be a case of where the cure is worse than the problem.
The last option is just to practice more until you learn to shoot more accurately with the larger gaps. After much experimenting, this is the option I finally ended up using.
Longbowarchitect’s post above is also correct regarding where to focus when using a gap.