Debit order running for two years with reducing

I have stopped using this credit card two years ago (Jan 2014), limit R800). A debit order is going of my bank account since then. Two year later the outstanding balance is R809.00. When I called the Customer service they said because the debit order was less than the interest + Service fee on the account. The debit order is for +- R80 per month X 24 = R1920 paid for the last two year and I still owe more than I did 2 years ago??????. In two years the interest and service fees was more than R1120 on an facility of R800?? Seems to me that Nedbank pulls their debit order calculations out of a lucky packet?