Interest rate charged incorrect on Overdraft

I have a personalised interest rate of 13.25% annually on my Cheque account for my OD. Interest on my account has always been charged at this percentage. Yet for the month of May 2015 FNB decided to charge me at an interest rate of 20.75%.<br> <br> FNB, I would like to know why was I not charged at my personalised rate of 13.25%. <br> <br> I never exceeded my OD limit at any given point of time on my cheque account.