It just means whoever owned the car before forgot to cancel their subscription, and they don't pay close enough attention to their checking account or Sirius account to realize they are still paying for your radio.
Thats what I am assuming too, because on my ONstar it said the expiration date was 9/13/08 and I checked again on 9/14/08 because it never stopped playing and low and be-hold the new expiration date is 9/13/2010