There might be several reasons for this, e.g. if you as breeder have forgotten to report ownership changes for registrar, ownerships have not been updated to register.
You can transfer the ownership in MyCats together with the new owner. Instructions for ownership change: Ownership change
if the new owner is not member of any club, the change will need to be done with paper form.
In case you do not recognize the extra cat contact the MyCats support.