Because I live very close to hubs (EWR and IAD), those minimum 500 miles trips add up; I.e., nearly half of my total flight count. If my average one-way distance is 3000 miles (2500 + 500 min), I have taken nearly 1000 one-way trips on UA. This means that 0.5 million of my 3 millions BIS come from those 500 min miles flights.
Cheers!