In my experience some of the older settop boxes don't yield first class video. Also, I suspect that Comcast tinkers with the compression ratio depending on the popularity of a feed. Thinly watched channels will be more highly compressed in order to make room for the popular stuff. Movie channels are consistently better than obscure stuff and the movie channels have more consistent lip sync.

Finally, there is still a mixture of SD and HD channels. Some channels have both SD and HD feeds. If you are not paying attention, you might be watching the SD version. The latest boxes allow one to automatically switch to the HD version, if available.