My guess is that they are trying to leave early to find a hole through the weather going from ATL-DCA. The weather up here has been screwy and lots of departure/arrival fixes have been opening and closing all day. Perhaps they want to make sure they get the flight out rather than miss their weather window and risk a massive delay or cancellation?

In other cases, I've seen "early" ascribed to flights who's actual flight plans versus planned indicate a shorter flight enroute.