On a sectional chart legend, the airport icon with a solid magenta or blue disk is labeled as "Hard surfaced runways 1500 ft to 8069 ft in length". This seems like an extraordinarily arbitrary number to use as the upper limit before a different symbol is used. When I convert this to meters, or yards, or furlongs, or any other common unit, it doesn't correspond to any even number so it doesn't seem like a conversion from another system of units. What is the reason for this unusual dimension for a runway length?