How does Cox measure data "usage"?

I've been dealing with a spotty connection for the past few weeks, well, few months really, but the past few weeks have been exceptionally bad. 

I've noticed that during the periods of Cox network failures my data "usage", as shown on their website, is significantly higher than normal.  During these periods it also deviates significantly from the throughput logged by my gateway, which is both fairly consistent day-to-day and more representative of the expected network load. 

My assumption is that this is due to retries initiated by the modem.  There have been a huge number of un-correctable errors logged by the modem across a number of channels during these periods, so that seems to support my theory.

Does anyone have any insight into this?