@ Garfield2010
I assume that you've had the Data Product for a while, (Else if it was your first month, then you are experiencing the dreaded "No Rollover Policy" of Cell C.)
If this was your 1st month, there is some good news, as the 1st months usage is "supposed" to be gratis, to accommodate mid and late month in-store purchases. (I have asked Cell C Reps to substantiate this claim by them - to date, no such luck)
What was the Upload amount of Data? As you have to add both the Download & Upload to arrive at the Total Data Usage.
And the Cell C Representative should have been clear that the "indicative" measurement could only be out by a few Megs (max 10MB for each 1Gig). Even though I find it pretty darn accurate.