Video in the Netflix app on X1 is streamed over the internet. For this reason, if you do not properly exit the Netflix application, it will continue to use your allocated data as part of your Xfinity Internet data usage plan.
This is expected behavior. However, Netflix on X1 will never play more than three episodes automatically without stopping to confirm you are still watching.
To determine what amount of usage you can expect for a given hour of streaming in this situation, please see estimates on the Netflix website help.netflix.com/en/node/87. Netflix estimates about 1 GB of data per hour for each stream of standard definition video, and up to 3 GB per hour for each stream of HD video. On this same Netflix help center article you can also read Netflix recommendations on how to control your Netflix data usage.
Please make sure you exit out of the Netflix application on your X1 TV Box before turning off your TV. This will ensure you are no longer streaming video and consuming your data bandwidth.