When It was given a commercial halt of intermediate stations, Duronto Trains used to run Non Stop (without any intermediate commercial halts). So, as other Duronto Trains were not having intermediate commercial halts and this train was given commercial halts - it's category was changed to Rajdhani to keep the fare high.
Back then Flexi Fare were not introduced in Rajdhani/Duronto/Shatabdi Trains and since Duronto trains were considered Non Stop the only option available was to convert it into a Rajdhani class train as SR wanted high fare.
Now Duronto Trains have intermediate...
more... commercial halts and Flexi Fare so this train retaining Rajdhani tag becomes a questionable issue.