I believe comcast cuts the picture size down to save juice so they can pile more connections on top of each other and then turn around and charge for a better service. This picture being cut off the sides got really annoying when they forced us to have boxes on each TV so they can charge people who use more than two extra TVs. I went from having HD full 6:9 ratio down to 4:11 with sides cut off on all TVs and I lost my HD.
I pay for HD on the main box but I cannot tell the difference except the picture is smaller without the sides being cut-off so I can zoom it in and get some use out of my 6:9 flat-screens, but I still get part of the screen cut off, this time on the top and bottom.
So when you hear that it is the station that is causing the picture to be cut off, as many stations offer different screen sizes, it is comcast whom funnels the picture size so much part of the picture is cut off.