Hi,
Could it be that your device has been reset or data traffic got somehow deleted during the month that the count was still going?
Yes, there are little mismatches from what the modem says and what the operator should, because some of the services are not being counted into the process of data traffic, but those are not that big.
Could you try maybe resetting your device by the end of the month and try running it clean for the whole month or at least half of it with only things that you need and then clearly give the diagram of operator data usage and what modem shows you?
EB.