What this chart does is visualize the results of poor trigger control, typically the primary reason for poor shots down range. If you can get the student to actuate the trigger properly their accuracy will dramatically improve. After trigger control is 'corrected' the chart allows the student to work on the other aspects of their grip to eliminate incorrect behaviors.
Sight alignment is easy for most folks until they start the firing sequence, during the firing sequence the student is not likely looking at the sight picture any longer but trying to see where the round will go before the round goes. This results in the previously 'on-target' weapon now drifting off-target, where the other incorrect behaviors the chart is designed to visualize for the student come into play.
If shooting a gun was easy, anyone could do it and do it well.