Firstly I agree with your point that at present there are no towns in Bihar which can be compared to Patna.
But I assume about 25% (a rough approximation) of Patna's revenue must be coming from other towns of Bihar.
Example: Patna has daily connection to Banglore, while only 3 weeklies originate from Bihar to Banglore other than Patna i.e Bagmati, MFP-YPR and Anga. So many people travel from parts of Bihar to Patna for catching a train to Banglore. So these people add up to earning and footfall of Patna...
more... rather than their home town. Similar example can be taken for trains to Mumbai/Surat/Ahmedabad/Goa/Indore/Hyderabad/Chennai etc.