Your personal feeling can be wrong here , because Shatabdi And Duronto don't require P7 cause:-
1. Shatabdi Express has less coaches and Needs More speed and less Halts.. P5 does the job effectively.
2. Duronto is a NON stop train , which doesn't require Much Deceleration and acceleration , P7 has Braking and Faster acceleration , As duronto doesn't stop much as compared to Passenger trains, so justified to give p7 to passenger train then giving to Shatabdi and Duronto.