Paid more for the item ordered as the amount I agreed and accepted when I purchased the item

I only had one order on my account a Dell Monitor, repayment amount was suppose to be R153 per month. I only notices now that I paid R192.84, I would really like to know why I paid more than the amount I accepted when I ordered the item. I just settled my account but I would like to know why I had to pay so much more, and would like to be refunded for the extra I had to pay monthly and on my settlement amount - because it was not the agreement I accepted when I ordered the item.