High Instalment on my bond account

I'm having a bond account with st .bank to the value of +-R400000.00 and the monthly instalments is R5500.00 which I'm not happy with.To my knowledge the interest rates for bonds is 10.25% and I should at least be paying +-R4000. 00. Yesterday I went to their branch and phoned the home loan department and the explanation I got from one of their consultant was unsatisfactory. Could you please help because they say my bond is in arrears of R500. 00 Regards