Typically, by taking off to the west from ATL, flights to the DC area airports can be as much as 100 flown miles longer than the linear "airline distance" between the reference geographical coordinates of ATL and those of DCA, IAD or BWI.

I think somewhere in the actual published rules for the SM program there is a reference to the actual published distance between city pairs. I think the mileage that DL uses for SM credit is very close, if not 100% accurate, to the actual linear distance between airports. If you are interested in checking this, there are a couple of tools you can use.

First, go to http://www.airnav.com, enter the airport codes, and this site will give you the geographic coordinates of the airports and a lot of interesting facts about the airport runways, elevations, etc.

Then, to calculate the distance between two sets of coordinates, there is a great tool on the Federal Communications Commission's website, found at

http://transition.fcc.gov/mb/audio/bickel/distance.html