Distance from Point A to Point B is fairly self-explanatory - it does not include "out of route" miles for a longer path on your journey. For example, the distance from DTW to HKG is NOT 8100 miles, its 7850...however, if alternate paths are needed, then the trip mileage increases, but the distance does not. Any attempt to argue this with the airlines is futile.

As an fyi - the airlines use the "Great Circle distance" as their measure...
http://en.wikipedia.org/wiki/Great-circle_distance