BSNL broadband 500c plan — carrying forward of data used to next month

RESPECTED SIR ,i have a broadband connection and in october ,i saw that there is a column in which it is written "september unbilled data" now this data is added in my october bill and thus it is beyond my free usage limit so i have to pay for that ,while i have free usage data left in september . so why i should i pay it in october.
And above all if BSNL donot carry unused free data to next month then why they are carrying used data to next month.