Odd that they would say that since ppm doesn't really work in a linear way like that. Otherwise we'd all be using 500 ppm silver (I've tried it and it didn't work at all). But ppm is confusing enough, so it's no doubt easier for the mfgs to let consumers have their own perceptions than explaining how sometimes less of the active ingredient is more powerful. Try selling THAT to the pulic.
High ppm doesn't work as good as it should because the particles get bigger (agglomeration) if you go much above 20 ppm. They have to add stabilizers to keep it in suspension.
To take a huge dose you can simply drink a jar of the 15 ppm. Which costs nothing but the distilled water if you have a generator. The particles are much smaller.