First I say my calculator gives the *exact* mileage that UA uses, then I go and say it's more accurate than calculating it geographically. I just can't stop stuffing my foot in my mouth today.

You're right - that site is very accurate. I assume they use some version of Vincenty's algorithm (I could be wrong), which is one of the more accurate ways of predicting distance on the earth. Most sites don't use this, however, because it is more computationally intensive. The typical way is to just use a simple equation which either assumes the earth is a sphere (not very accurate), or else ellipsoidal (semi-accurate).

The point of my calculator was to eliminate the ambiguities and just pull the data direct from the airlines. Unfortunately, it seems that the airlines don't even have the right data in all of their departments... hopefully when UA updates united.com, the new data will be exactly matching what MP uses. That's the hope at least.