Here's one explanation: He was making the same mistake as golfers make. He's taking his eye off the "ball." Right handed shooter gets excited about seeing if he hit his target and pulls gun down and to the left. Watch where the bullets hit.

The only other thing I can think of is that his gun isn't sighted in properly, but a good marksman would be able to see that and compensate.

meh, they know it's out there, my friend bo has a vhs of this. You might remember him james. He's expert marksman instructor in the Marines.