I think mostly the mileage is a general hope for a time frame. Because engines, well most engines, don't come with a running meter on them, they use mileage.
It's very likely for an engine to enjoy low mileage oil changes when the hour meter gets high, stop and go traffic, short and slow trips. But if you like to drive and enjoy taking the slower back roads, keep the power needs down, but run up the miles... why not get a little extra out of a good oil? Engines like to run at medium power steadily for miles, but short and choppy, or pedal to the metal uphills and high speeds take their toll.
