Explanation to the two different balance on my student loan

<p>12/05/16 I called customer care and I requested my student loan balance Sanela sent me a statement the outstanding balance was R13161.05 as of the 11/05/16.On the 13/05/16 I paid R10000 and R664 was paid15/05/16 for interest part of this money also pays for the outstanding lump sum.On the 26/05/2016 I requested a the new balance because on the 25th interest would be added and I wanted to know the exact amount.I was surprised of the new outstanding balance which was a difference of R664 if I didn't add the monthly interest.I called Standard Bank Collection on the 27/05/16 for an explanation and Thabi and Thembi couldn't help me so I was transferred to Basani who couldn't help and she was rude and she referred me to go the closest branch.I called again and Nelisiwe tried to help me but I'm still waiting for her response,I went to a branch and I showed them the two different Statement they couldn't help.I'm unhappy with the services and I want an explanation to the added R664?</p>