NDLS is One of the BIGGEST and Crowded stations in India. So for smooth controlling and less congestion in and around NDLS , some trends or we can say "Rules" are made for convenience. Alighting 5-10 minutes delayed is WAY BETTER than derailing or getting Stuck in Traffic Jam (Railways Jam).
NZM yard is also nearly full of LHB trains but still for convenience of Passengers , they can adjust one more AC Rake.