Cell C
Overcharged on data

I am being overcharged on my CELL C account. After a couple of months having quite high data charges I decided to install a reputable data manager app to keep me in check.
August was the first month I started using it and I used in the vicinity of 1GB of data. (according to the app)
To my surprise when August bill came it was higher than ever.
I went through the itemized billing and saw that by the 17th I used up my 500mb included in the contract. This sounds about right.
However, the data manager app suggests that I used 430mb for the latter part. According to CELL C's account and using 100MB @ R15 rate, the bill suggests that I used 2.3GB.
I understand that there could be slight variations in the app and CELL C but not this margin.
I therefore now know that the previous months have also been overcharged and I need to know what my options are of stopping this cycle.