Subscriptions, not subscribers. Many subscribers have multiple subscriptions, because each radio one owns needs a subscription of its own. So the listener who has SiriusXM available on their factory-installed car radio needs another subscription for any radio they may be using in the home or office.
Also, at any given time, hundreds of thousands -- maybe more than that -- of vehicles are sitting unsold in dealer lots, their SXM radios activated but listened to by no one. All those radios are accounted for as subscriptions by SiriusXM in a weaselly, but technically legal, method of representing its numbers to the SEC.
So satellite radio is likely a bit less of a hit with the American consumer than the rosy numbers the company puts out for public consumption would indicate it is.