I've complained the same for quite some time...and I'm a 20+ year customer of Cox. It's probably different from ATT.
I've never exceeded 1000GB until last month...and Cox had issues all month. We were down as long as 8 hours at times. There is no way we exceeded 1000GB. In September we used under 600 GB, October about 800GB Normally it's about 750-800GB.
There is no consistency in their monitoring. Days I expect higher, it's often lower. Days I expect a lower result, it's higher. I don't vary my schedule and viewing habits much either...it's just my wife and I. Sometimes I do so just to see how the monitoring responds....it's inconsistent. All I can say.
I don't know how Cox measures, but I question the veracity. Way to inconsistent in it to accept it as accurate.
The recent "system upgrade" seems to have produced even higher numbers. Expect worse.
Good luck.