Todd Jarrett sez: hold the gun 20% tighter.
In all seriousness, your friends are shooting low with a DA gun because they're not good at shoot with a DA gun. Functionally, there is no difference between shooting a DA gun or a single action gun in that it's the same thing: sight picture and trigger control.
The difference comes in the "trigger control" portion of the event, where people with sloppy trigger discipline get rough on a DA trigger because they're not used to the extra pull weight. Instead of pressing the trigger, they end up jerking or slapping the trigger resulting in poor accuracy.
Also, as an aside: that graph that shows "if you're hitting here you're doing this" is great...but was originally written for one handed shooting. It's not applicable when you're using a proper (emphasis on proper!) two handed grip.
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