What does Cox use to measure internet data usage? Seems to be over-estimating

As a test to whether it's better to use TiVo and record TV shows, or use streaming service via Cox and our internet data, I found a discrepancy of at least one gigabyte (1gb) per day, sometimes more.

Using my Asus router, I accounted for ALL clients sending/receiving data through the router, and have been taking photos of the daily difference between my router's numbers, and Cox's Data Usage estimator.

To difference benefits you, Cox, in what could be as much as 100gb per month.

That's nearly 10% of our monthly allotment.

Needless to say, we'll be returning to TiVo until Cox can tell us how they estimate, record and bill us for data usage.

Any other Cox users see this as well?

So, how DO you measure our data usage, Cox?