FNB overcharging me on Bond interest.

I have a Smart Bond Account with FNB which I took out in August 2012. I have recently noticed that I have been overcharged on a monthly interest. I queried this with them, specifically the July interest.

All FNB has done so far is send me an interest calculation formula and that is definitely not addressing my issue. I have back and forth email exchanges between me and them for almost a month now.

FNB is overcharging me and they cannot explain why that is. I am sure what they are doing is against the law.