Billed For Arrears interest while my Account is up

I opened a silver credit card with African Bank in May 2015 and been paying them well since via early debit order, My account is well up to date but to my surprise when I checked my statement for the month June and July I noticed I was charged an arrears interest of about R360 respectively, I phoned the call centre to ask about this and they put me through to the credit card department where it was confirmed that my account is up to date and they don't know why I was charged arrears interest and I should check back in 7x days I did that after 7x days but when I called the agent told me No African bank uses an incorrect wording in their statement, What is that? to turn around like that I did enquire what arrears interest mean in their own terms and they agreed they have been falsely charging me now instead of reversing the funds I being told its just their incorrect use of the English language...