That came out in the early days as a work around for XM radios dropping signals around tall buildings. Sirius satellites move, and they have three in orbit, so there's a good chance you have at least one above you. XM has two that are stationary, one east coast, one west. So as you move, you could lose the signal more easily. NYC is one example where XM had to add repeaters to keep subscribers happy. I've only found one area in all my years of driving that I could consistently lose signal with my Sirius unit, it was down a county road in the woods of Massachusetts, for about 5 miles. Once I got out of that area, everything was fine.
Found this article that gives a basic overview of the two sat systems...
http://www.johnweeks.com/steps/siriusvrsxm12step.html