What we have to remember is that you never have to renew your SIN number. Back many years ago—I know this is true for colleagues in the office, and probably many people around the table—you got your social insurance number when you got your first summer job, and it was actually your employer who got it for you. Some people say they got a number every year.
So it could be duplicate numbers, it could be people who have left the country, it could be people who are deceased. Again, the links with the vital statistics would give the registry the information on people who are deceased and whose cards can be cancelled.