Charged wrong interest rate on bond

I took a mortgage bond with NEDBANK. the gave me a main banked interest of 6.15% if I open and account and move to NEDBANK or deposit R5000 monthly. I decided to deposit R5000 Monthly however the bank is charging me 6.4%. I contacted the bank's customer complaints department, however I have not received any help to date. The bank's reference number for my complain: CRM ***4.