Debit order R21 more that last month

Could you please explain to me why the debit order that went off by bank account today, was R21 more than last month and the month before? (I haven't upgraded - I am only due for an upgrade in January again.)<br> <br> I signed a contract for a debit order of a certain amount and THAT is the amount I will pay.<br> <br> I will also be writing to an authority on this matter.