To give you the longer answer, the original DirecTV HD offerings were in a standard called "MPEG-2" (don't worry what that means) and had to appear in the MPEG-2 receivers on a seperate channel number. DirecTV decided to use the 2 digit numbers higher than the possible local channel numbers (70-99) for these.
The newer DirecTV HD content is in MPEG-4 and replaces the SD content on the same number (if you set your box right, the box know to show the HD feed of ESPN on 206, rather than the SD feed).
However, some customers, particularly bars and restaurants that bought DirecTV just for a commercial application to NFLST, still have only the MPEG-2 box and dish. So the "legacy" customers still get the tiny amount of HD content on their old set ups. If you have the modern set up, the only reason to use the old feeds is pointless PQ discussions or during the occasional rain storm when one sat might be rainfaded out and the other not.
Eventually, DirecTV will drop the dual feeds and reclaim the bandwidth for more channels. This requires every HD customer to have an H-20 or above receiver and the appropriate 5LNB dish.