There seems to be no precise answer to that. The airlines use various figures for city-city distances and booking engines use various distances, and they frequently don't agree.

UA, for example, uses ITN as its booking engine, but UA's system allocates miles that can be quite different from what ITN reports. Sabre's idea of what the distances are seems to be almost (but not always) the same as UA's.

It's a mess, and I wish they'd just decide on one way to calculate it instead of the three or so different ones going around.