I use the "data usage" from settings, however, for the last couple of months, it's convinced that I've about 1.3 GB of usage even on the day that the usage reset.
Currently, the breakdown of mobile usage shows usage totalling 38.09 MB, whilst the total reported as used is 1.49 GB, which is nothing like the same total.
whilst I'd love to understand the cause, I'd just be happy knowing how to clear this block of data usage out, as waiting for the month to roll over hasn't worked.