FirstRand Bank pay 0,25% on "overpayment", charge 13,95% on same "overpayment"

FirstRand Bank administers the now defunct Kulula Credit Card. I have started paying extra in order to settle this debt sooner. However, the way FirstRand handles this is to NOT reduce your outstanding balance. Your overpayment is allocated to what appears to be a personal "Suspense" account. On this balance you are paid a whole 0,25% interest. You are then charged 13,95% on this same amount as it forms part of the outstanding balance. ***** in its most "legal" form.