Internet data usage is unusually high

I have been an xfinity internet and mobile customer for over 7 years. I have never paid attention to my internet data since I never exceeded the 1.2TB cap, until August this year. It suddenly doubled my July's usage of ~800 GB to over 1600 GB and reached 90% by the 22nd of Sept. There are no new devices added to my network recently and have been using the internet same as always (no gaming etc.). I started to monitor my internet data usage with my wifi router and compared it with the xfinity meter. I found the xfinity meter number is shockingly bloated. For instance, on 9/28, my router shows <3 GB usage (I turned off modem for most of the day to save the data usage) but a live xfinity agent told me I used over 9GB for the same period, he admitted there is error on their meter, and opened an investigation.

This data overage has caused us a lot inconvenience and my family was forced to use more mobile data and got data overage mobile bill too. I hope xfinity can provide a clear explanation and solution.
P.S. We have no need or interest in the unlimited data plan based on our normal use.