More money due after Settlement paid on loan

I requested a Loan settlement letter in December. I paid the settlement amount on the date it was due and they still deducted an installment the next month. I did a reversal on that premium and then Nedbank says I still owe them that amount before they can close my loan. So why must we then get a settlement amount and pay that amount and then still owe them money? I think they just see someone that is willing to pay and think, lets nail this person for more. No one can explain to me why I still owe Nedbank money after I paid settlement amount.