There’s a difference. This isn’t chosid shoteh and this isn’t over-paying for nothing.
Fact 1: You want to keep a certain level of kashrus.
Fact 2: Some OU products adhere to that level and some don’t.
Fact 3: You don’t know which OU products adhere to that level.
Fact 4: Rabbi Heimish does.
Fact 5: You pay extra money to buy the OU products that are also certified by Rabbi Heimish.
It might look like Rabbi Heimish is just putting his sticker over the OUs (which he is) but it is more than that- he is certifying that he agrees with the OU about the kashrus of this particular product.