My friend I'm only speaking for NY, not Philly, which is at least 2 1/2 hours from my place in NY by car. I doubt they would discount tickets from Philly as opposed to NY, with NY and DC being much bigger business centers. I just think your location wasn't their target demo, so it may have been cheaper for you to drive.
I am only speaking for us who traveled from NY to DC ( i also went to school IN DC and came home to NY every weekend).
And obviously you drive a smaller car (my car in college was a sedan that took 80-100 to fill up @ 93 octane) and don't have to deal with NY to DC tolls which add on $40 one way (Baltimore Tunnel, Perryville, Delaware Bridge, Holland Tunnel, Delaware State,NJ Turnpike. Just the tolls alone is more than your entire trip.
I guess it's better for NY/Boston travelers than Philly or Newark travelers.