The manufacture also reserves the right to package a lower quality product. I've bought generic cereal that I'm sure was Kelloggs, most of the time I couldn't tell the difference - every once in a while there was a slight burnt taste. Still food safe, but not as high quality.

But why should we accept that?

Because even though the product might be from the same supplier, it is not a product of this brand. There is a big difference.

The customer-expectation of a brand-product is based on the perception, the history and the presence of that brand, a well-crafted association and experience that is funded with the profit of the product.

The private-label product is cutting this funding. It will not finance the supplier-brand and it sets its own expectation and quality-thresholds on the product. The supplier accepts that and waives his control to get the volume-order and fill production-gaps in its factory.

In this kind of deal there is no direct relationship between the original brand and the private-label product anymore.

Edit, for clarification: It's not said that the potato-chips will be exactly the same. The minimum requirements of the brands may not match with the minimum-requirements of the private-label, i.e. for the private-label a different grade of potato, oil, salt might be used.