Cox Data Usage Notification

I recently received an email stating the fact that we went over our allotted data plan.... When reviewing both the Cox Data Usage page and matching it with our routers Traffic Manager - Traffic Monitor - Cox's data was substantially over of what our router recorded. In fact not until this month did even the Cox usage page indicate over 200 GB of usage. Nothing has changed in our usage and our daily traffic Manager data does not even come close to what Cox has recorded for the month. Can you explain this to me.