Supermarket own brand products are often of a much lower quality than branded versions, but sell well because they are cheaper. There is a quality/price trade off with a lot of products and a demand for those on both ends of the spectrum.
Some store brand products are premium equivs while some aren't. (Some stores even have two brands so they can compete with the premium with the same quality at a slightly lower price and at lesser quality at a significantly lower price.)
There was a big Harvard Biz Review article about store brand strategies several years ago.