In July, my reported data usage spiked over 1.2 TB. The only reason I know is because I received an text alert when it was nearing that amount. And then there was a $40 charge on my bill with a one time per 12 mth overage credit of $40. I have never exceeded that data cap before and my monthly usage for the past year averaged right around half that amount. Coincidentally, I was notified in July that the Xfinity Internet service was being upgraded and would be for awhile and there would be associated outages. Interesting coincidence. Side note - I would think there would be an availability credit if they are going to do maintenance and upgrades, and yes, more than once I had to connect to my mobile hotspot for Internet.  

Nothing changed regarding my usage in July. Contacting chat was worthless as all they did was try to get me to sign up for Unlimited Data or switch to Xfinity equipment.  If Xfinity can't explain their data usage tracking error and correct it on my account within the next week, I will be switching to a different ISP. Quantum Fiber is available in my area; is cheaper, and they do not have a data cap.  Google Fiber also just placed fiber throughout my area and that will also be available soon.  In addition, I think I will also look at alternatives to Xfinity cable.  So, I may no longer be an Xfinity customer at all in a few weeks.

P.S. Googling unusual Xfinity Internet data usage spikes returns a lot of Xfinity customers reporting the same. Xfinity - you clearly have a usage tracking issue!