We never get data plan of complete month. We get data plan of 28 days, 58 days, 84 days instead of 30 days, 60 days, or 90 days. Have you ever thought why what is the reason behind this ?
The month usually has 31 days but they give us net pack of 28 days. The 3 days are left. There are 12 months in a year. The leftover 3 days X 12 becomes 36 days. Creating a 36 days almost a month.

EXPECTED:
31 days =1 month
1 year = 12 recharges

REALITY:
28 days = 1 month
1 Year = 13 recharges


Due to that left over 3 days , we have to do recharge 13 times in a year. The extra money of that 13th recharge from million of people gives them profits in crores.
So we should also go with the maximum recharge offered by the service provider as that may save you some more money than doing monthly recharge.