Yes this is justified as per rules.....because you booked from SDAH but your boarding point is Dhanbad..you will get the benefit/ entitlement of Dhanbad boarding passengers..... you cannot claim for sdah to dhn sector.
This is another way of earning money by Railways...many passengers of Dhanbad book tickets from Sealdah pay more money but board from Dhanbad. And to earn this money extra Dhanbad alloted less no. Of seats compared to number of boarding passengers from Dhanbad.