What happened to all my data?

I was allocated with 478.97MB data on 2015-03-03, probably pro rata as my new contract kicked in during March, which includes 500+500MB data.<br> <br> Yesterday I get a notification that it's all finished. How? According to my phone I used only 187MB since 2015-03-01, and I have probably never used that much data in a month, as I am on WiFi most of the time anyway.<br> <br> Please explain where this 291.97MB worth of Data went. A small discrepancy, even up to 10MB, is acceptable as the measuring protocols may be different between Cell C and the phone, but this amount is unacceptable.