Be careful to exit from any streaming app before using your remote to turn off your system. The app will continue to play otherwise and data usage will continue to accumulate even though you are not watching TV.
If there is a Comcast expert or employee or retired employee listening, I have a simple suggestion to avoid users potentially getting charged high data usage even though they are not watching TV.
Why not program the Xfinity remove "power off" button to "EXIT" and then "EXIT" in addition to turning off the TV. The topbox stays on so that the update can take place overnight, but in case the user forgets to exit from the app, no data usage will occur through the topbox modem.