Originally Posted by
MM_someday
It's ok, the great circle distance isn't a true measure of the actual BIS mileage either, it would have to be specific flight path dependent in 3D space. I just irrationally "rationalize" that the 500-mile minimums are a way to recover some of that lost miles spent circling around airports, taxiing in the runway, diversions from programmed path, etc.
500 mile minimum likely came from the practice in mileage earning based days when 500 miles was the earning limit for elites. Note this was used for many partners also. In the days I flew STS-SFO (66 flight miles) it help make up for the frequent fog cancellations.