I think the hand thing is a bit misleading in my opinion.
If you are doing the hand test yourself, the way most people grab their penis is with their thumb closest to them. Therefore your thumb is on your shaft, and your 4 remaining fingers are on the underside of your penis and on your testicles. I'm not saying that this is an incorrect way to do it, but most people dont measure from the underside of the penis, but rather from the top
So most people could probably add another hand on top of their thumb, since thats probably only 5 inches or so