Comcast is a company that has largely grown through acquisition, so not every market has the same plant design or capabilities. Some markets are engineered to have a 750MHz plant, some are a 1000MHz plant. Obviously the markets with 25% more RF spectrum to work with are going to be able to crank up more HD.
In general, Comcast is hoping to address most of this through what they're calling
Project Cavalry, which is the elimination of most of the analog channels they carry today through the deployment of low-cost digital tuning adapters to their subscriber base. For every analog channel they eliminate, they get ~38mbps of digital bandwidth (once converted to QAM) to deliver SD and HD programming. Actual channel counts they'll get into that space depends on how much compression they apply.
