[MaHuJa]

I suspect decoys in general have a "dud chance" given by the "cm effectiveness" in the database.

If 50% then on average every second countermeasure would do absolutely nothing. Except be a bright target to a human player.

This is only a theory of how it works though - other theories would be along the lines of how easily it seduces a torpedo to break lock with the real target, but I'm doubtful.