A question of a very basic type: How are the frequent-flyer miles between cities really computed?

For example, on a recent flight, I was credited with 3255 miles. Yet the 777 in-flight monitor listed the distance between the two cities at take-off as 3265 miles.

A difference of 10 miles.

I do know that there are lanes that aircraft must observe, analogous to 'skyways'. But if airline X is offering you k miles between cities x and y, is this the average of the possible routes, or merely the number of miles of the minimum-distance route?

Does someone know? I know a difference of merely 10 miles is not much, but I feel that if this is happening to all travelling passengers worldwide, the airlines are saving many a reward trip.

Thank you.