I use my own router/modem and it has a traffic meter. The traffic is measured for both up and download traffic. So all data passing through router is recorded. However, the amount of data used is usually half of what Xfinity lists on their site and app. I show 11.5GB this month (meter reset on 7/1) and Xfinity shows 21GB.
Why the difference, and what can be done about this?
Try calling Comcast Security Assurance they are the people that deal with meter questions. Ask the phone rep to put you through to them:
The Customer Security Assurance organization has been established to ensure a safe and secure online experience for Comcast customers. This team is a dedicated group of security professionals who respond to issues pertaining to phishing, spam, infected computers (commonly referred to as bots), online fraud and other security issues.