So, a "flier" is a "bad shot", yet there is such a thing as one, but no such thing as the other, and a shooter can take action on one, but cannot take action on the other, though they are the same thing in different terms. This would be ridiculous semantics, except that now I think I see the terms are being confused.
A "bad shot" says nothing as to whether the shot is "bad" due to shooter error, or to a bullet's deviant trajectory. "Flier" does not imply there was not a reason for the deviant flight path. I don't know anyone who thinks that a flier cannot be explained WERE THE REASON BUT KNOWN. Sometimes the reason can be determined, and sometimes it can't, but there's always a reason for a flier even if it cannot readily be explained. A flier is a type of bad shot, but a bad shot is not always a flier. They are not one in the same.
I understand that with a quality rifle and quality ammunition a marksmanship coach HAS TO assume any shot outside of expected dispersion is a "bad shot" due to shooter error for the sake of instruction. But that does not mean that there is "no such thing as a flier" in reality. You may want to use another term for it, but most understand a "flier" is a "bad shot" that is not due to shooter error.