Well how Box Office is tracked in India. That's 1st thing you need to know. Every Tracker (We have only one I.e. BOI) has its sources I.e. Distributors. They report the figures to BOI and BOI report them to us. Though there are various Internal Controls which BOI follows.
Now why BOI is to believed because I tried getting figures from Distributor and whatever I received was similar to BOI. Though there are few differences to the extent of 1-5% which can be ignored.
Rentrak tracked few films in India over last two years & their figures were same as that of BOI. Rentrak tracked Mohenjo-Daro in India and as it happens there was no discrepancy in that film's figures.