Highest graded known copy of any one comic is not the same as the concept of 'high grade'. Grades are based on the number of defects that a comic book has. A comic that has enough accumulated defects to result in a grade of 5.0 is not, nor has it ever been, high grade. Can it be the highest known graded copy of that comic? Can it be as you say, "high given its own distribution" Sure, it can but that doesn't make it high grade. If it were high grade, it wouldn't have enough defects to knock it down to 5.0.