Iska Sidha sa Matlab hai regarding 12903:
GT mail have a DIFFERENT quota (RLWL) from Surat station. And for people booking from BCT have GNWL quota under them.
Now consider All seats in GNWL i.e Mumbai to Amritsar are full (regret status) and also the Next quota i.e Surat quota (RLWL) is also Full. But Who will use the seats that are reserved under Surat quota between Mumbai and Surat ? Railways Can not run these seats Emtpy from Mumbai to Surat. it will be a Huge Loss. So thats why Even...
more... though you have got PQWL 77 , but this doesn't means all the seats of Surat quota (i.e poeple who have booked from Surat to onward journey) is Full. Some of these seats are left vacant between Mumbai and Surat due to less booking. So ur ticket got confirm after Charting. This is one type of "Berth Multiplexing".
I think i have explained in lucid manner. Still any doubt, feel free to ask.
Thanks.