in my experience (as long as I don't include many back-/forward segments in the USA), it doesn't differ much in the total. When Amadeus charges city-to-city miles (example: all New York city airports EWR, LGA, JFK show the same mileage distance) and other systems don't (they show individual airport distances), on a rtw trip going mainly in just one direction, it equalizes out (more or less. I never noticed dierrencies of more that 40 miles in the totals).