Do you think the hsimish companies maintain their own canneries etc? They contract. With many of the same companies that manufacture canned goods with an ON. They just apply the heimish label for a specific run and you get to pay a higher price for the privilege. A few years ago a friends father insisted on paying a premium. For a bag of sugar with a heimish hashgacha.
During Pesach the bag tore and underneath was the Domino bag with an OUP