The reroutes were designed to give Vienna more trains and to speed up travel from the ends of the Blue and Orange Line. Before the inner Green line fully opened, getting people in from Greenbelt was also a factor. Even now on July 4th, they short some trains to provide the most service where it is needed (western branch of the Red Line, southern end of the Green line).
Originally, the Yellow line was to go to F/S and Blue to Huntington. When Huntington was ready to open in 1983, they were short on cars, and the Yellow "Shuttle" from National Airport to Gallery Place was really not carrying as many riders as they hoped, so it was decided to swap the Blue and Yellow lines temporarily. The Blue Line would end at National Airport, the former terminus for both lines, and the Yellow line would continue south. They would swap back when the "Yellow" line opened to Van Dorn Street. Of course, no one was planning this, so the trains when they had roll signs had HUNTINGTON on a blue background and the flip dots on the Bredas originally associated the code for HUNTINGTON with the Blue line and signed it as such. The maps also indicated that the under construction segment to Van Dorn and Franconia-Springfield was yellow (though Huntington was marked as yellow, too). When Van Dorn Street opened in 1993?, it was served from the Blue Line by the start, though I am not sure why. John, can you help us here??? I agree, it probably would make more sense to have Yellow go there, though using the downtown line names on July 4th makes more sense, since Blue Line trains usually don't go to Gallery Place.
An interesting note, following events at MCI Center (Gallery Place), there are 3 Blue Line Specials from Gallery Place (they might originate at Mount Vernon Square) to Franconia-Springfield. That "service" does in fact run outside of July 4th.