I was thinking about this while drinking orange juice. It's currently orange season, and there city is peppered with fresh-squeezed orange juice trucks. Literally, it's a guy with truck full of oranges and a gas powered juicer. The juice is fantastic; fresh, pulpy, and with an almost green taste to it. However, it tastes absolutely nothing like any store bought juice I've had, no matter what the label says.
So I was wondering what other foods there are where the version you make at home, and the version you buy in the grocery store are so completely different that you would have trouble telling that they're nominally the same food. I'm not talking just about quality - my spaghetti sauce is infinitely better than Prego from a jar, but you can tell that they're both spaghetti sauce.
Another example would be mayonnaise. Homemade, fresh mayo made with egg yolks and olive oil and jarred mayo from the store are so different as to be two different types of food.