There is a thingness about familiar products. Their printed labels say "genuine" but any certainty is conferred by our own eye that recognizes the brand elements. We know at a glance that it is what it says it is. There's also the guarantee from context. Powdered mustard is less likely to be counterfeited which is why you can seldom purchase it out of the trunk of a car or from a furtive gentleman in a trenchcoat. You have to go to a grocery store, and who is more trustworthy than a grocer? It doesn't hurt to know the royal family likes it too.