ZD,
Some of those products (maybe even most) are manufactured differently for the heimishe companies. Most heimishe products are made in big factories which are kashered for their production. Usually they use separate packaging, but sometimes they save on the cost by using the same packaging and just adding a sticker.
I should have pointed out in my earlier post that many who keep CY do so not as a chumra, but as the actual halacha. Many poskim did not agree with R’ Moshe Feinstein’s heter.