The company says that the traditional internet isn't used for the streams.
The practice of limiting the amount of content that someone can download is a practice that more and more internet providers are adapting, and one that's certainly frustrating to power users and other people who stream and download more than the disappointing limit.
The folks at Comcast have some good news for people streaming Xfinity content through their Xbox 360 – none of your activities will count against your data limit. Comcast says that this is because their service eschews the traditional internet and goes through Comcast's own private network. This doesn't apply to Xfinity content accessed through a computer though – that's still capped normally.
Source: Home Media Magazine