I work in a local taxation office and we get alot of people saying things simlar to this but they are rarely correct. You may be paying differently because you have different banded properties or (more likely) you are using different methods of payment. Payments for council tax are usually made over 10 months but they can be extended two 11 or 12 or shortend for any munber of reasons.
To clear up, you might as well call your local tax office to clarify the situation.