Home loan debit order wrong

I have a homeloan. I received a subsidy when i was still working. Im on pension for nearly 5 years. I DO NOT get a subsidy anymore. I pay the full amount out of my pocket. Some months I pay much more than my installment but then Standard bank still deduct on a debit order also an amount. I do not know for how long I tried to explain this to consultants. If I pay the correct amount or more WHY DO THEY STILL NEED TO DO A DEBIT ORDER. Can someone please assist with this. Some months my installment was for eg. R2900 THEN I PAY R3500 THEN THEY STILL DEDUCT DT ORDER R1000. Then the amount paid was R4500 instead of R2900( just an example) I normally transfer the amount out of my current account Thanks