There are probably a number of reasons for the surplus. One could be deaths, another could be people leaving the country.
As well, if we go back several years, into the seventies and previously, it was considered just a file identifier. Probably many of the people in this room got their social insurance number when they had their first summer employment and their employer applied for it. Some may even have received more than one. You would just sort of get these numbers. There wasn't a lot of control over them.
So there could be duplicate numbers in the system, and that could account for some of those dormant numbers.