Thanks for the explanation. I do know DBR is debited exactly at 0000 hours. However, it is still not clear if that amount which was debited was of the previous day or for the next day (considering 0000 as the turning point). I believe it is for the previous day.
For example: Let's say you buy a new ADTV connection today (11-Aug) and recharge Rs 300. Your package DBR is Rs 14.00. You are able to watch your television today (11-Aug) and your balance stays Rs 300. Only at the end of the day, at 0000 hours (Lets take this as 23:59:59 hours), it debits Rs 14.00 and your balance reduces to (Rs 300 - Rs 14 = 286).
I am convinced that the DBR reduction happens "after the day's usage" and not prior to the day's usage. Hence, as mentioned in my OP, if I activate new channels (say, worth additional DBR Rs 3), airtel instantly debits it when the package is changed. Hence, tonight at 23:59:59 hours, the DBR should be the 'old DBR' only, as additional Rs 3 was already taken by airtel during the day.
From next day onward, the DBR should be Rs 14 at 23:59:59 hours of 12-Aug, which is correct.
I still maintain that airtel is Double-Dipping the additional DBR amount on the day the channels are activated. Yes, it is a small amount, but considering total number of subscribers X plan changes per day, it will certainly grow to be a significant amount.