Possibly airport-to-airport vs city center-to-city center calcs. See #5 and further in the below.

http://www.flyertalk.com/forum/star-...-database.html

The UA mileage looks close to the great circle airport-to-airport mileage:

http://www.gcmap.com/mapui?P=phl-den,+den-sna

What's funny is the Star Alliance timetable has its own values, PHL-DEN 1555mi, DEN-SNA 940mi [sic]

http://www.staralliance.com/assets/d...arAlliance.pdf

IATA has its own official mileage (Ticketed Point Mileage), not sure if the US values are adopted from IATA:

http://www.airliners.net/aviation-fo...d.main/169776/