Good catch, malaki86!
I tested it now from my current location (Toronto) to Mexico city. The Summary in the Directions pane indicates 4,147 KM, but in the bottom right corner of the Navigation pane I see 3,153 KM.
But anyone who charges their customers per mile, do not rejoice ! The higher mileage may actually be the correct one, at least that's what Streets & Trips 2009 also shows.