This is true of a lot of materials that get recycled by melting. The recycled content nearly always has some amount of impurities that have to be offset by using virgin material, or using it only in an application where the flaws introduced by the contaminants don't really matter. Like those 100% recycled plastic benches you see from time to time. Generally though, it's one of the reasons it's so rare to see a product that has 100% recycled content. Steel, plastics, and glass are all affected by this. Aluminum is a notable exception.