--probably been a long time since your last chemistry class.

The difference, as Mrs. D point out, is that the potassium chloride designation is the compound that contains the elemental potassium. We cannot ingest alkali metals in elemental form--most are not even stable in that form (as the chemistry experiment scenario suggests), so that in order for us to get these into our bodies, we need to take them in a much more stable form in which they are part of a chemical compound. Our bodies can then break down that compound through digestion and use the metal nutrient, though in most cases our body complexes the nutrient into other organic (carbon-containing) compounds for use.

From what you're saying it sounds like that the potassium is contained in potassium chloride, but the ELEMENTAL value of potassium in a single does of that chloride is 530mg. That is the "nutritive" value you would get from potassium in a "dose" (the container likely lists per tablet or per two tablets) of this particular potassium chloride. The dose that gives you this 530mg of potassium is certainly overall heavier than that--not only is chloride in it but I'm sure there are other things, such as fillers and buffers, in it as well, in order to make it digestible.