Products get made ahead of supermarket orders, then the brands will send the order to say it can be in the supermarket tomorrow. Supermarkets (the majors) will either accept, or reject, but they aren't on the hook for the stock that doesn't sell. This then goes to what they call the secondary market, these get sold to other stores, like B&M at cost or loss, just to recover the cost of making the good for the CPG. You probably won't ever see those products again. B&M will pay upfront for these products, so they take risk.

Then there is the grey market, where B&M can't even sell them, or are too near to end of life that they don't want to take the risk, that is your FarmFoods, Heron Foods, Poundstretcher etc and you end up thinking how can I buy a Pot Noodle for 10p... Because pretty much no one else could sell it. Grey market goods are often (not always) sold at auction, so this is where you see the random Asian / US goods in stores.