I would agree with most, and your own diagnosis, concentration. I know when it is a good shot, and bad also. And when it is bad I know that I was not fully focused, but just winging one down range at the whole target rather than a small spot on the target. This sometimes happens if I have shot a couple of rally tight shots, I just expect the next one to fly to the same place and let my concentration drop, and what do you know, it is off