With positive passenger/bag matching rules, what happens to a bag that does not arrive with the passenger?
This is more a guess:

Think about the rationale behind the rule. A terrorist should not be able to send a bomb on a flight while staying at home. With this rule, a terrorist is forced to sacrifice himself by taking the same flight. Hopefully, most terrorists don't like this idea.

If the luggage wasn't put on board, or was put on another flight, the terrorist will notice this at the destination, and not before. Same effect, he had to be willing to sacrifice himself by boarding the aircraft.

So, the rule is not to never let luggage fly alone, it is about not letting it fly when the owner for some reason does  not take his flight.

If the luggage took another flight, it was for sure an error. The barcode scanner mis-read the barcode, the ID of the trays transporting the luggage was not read correctly, a computer error or whatever. Should not happen, but does from time to time.
If the luggage did not fly at all, it might simply have missed the flight due to technical / logistics problems. Or the barcode tag was ripped off etc.