Bruce,
Interesting point. For may trad archers who shoot more instinctively, the miss may be caused by many different factors but that is another issue for discussion later. A miss from this type of shooter is, in many ways, similar to an arrant throw by an MLB third baseman. How can that happen? This professional athlete has made that throw tens of thousands of times in his life and hundreds, if not more, of times each season. That said, misses happen, especially when eye hand coordinations issues come into play.
For archers who shoot the gap method, misses can occur by misjudging yardage and simply shooting the wrong gap estimation.
A very good trad archer friend of mine, who has attended Rick Welch's archery lessons, said to me that the left to right arrows we shoot are the arrows we need to be most concerned with. Those are form breakdowns. The high and low arrows are simply a misjudging of distance, which if you shoot archery long enough and happens to us all.
Dennis