I recall that Comcast does not count some streaming video services against the data caps. There were some (like Netflix) who complained this was not in the spirit of net neutrality. So maybe they are still not counting some of the video you stream, depending on where you are streaming it from? Just a guess.
Here's some more info on this: http://arstechnica.com/tech-policy/2012/03/net-neutrality-concerns-raised-about-comcasts-xbox-on-demand-service/.