Gosh. Those instructions are so hard to read. It should say (1mL / 21ppm * 1 gal), but it doesn't. My math worked out to be 262.857mL just like the rest of you.
I strongly advise for people to make their own solutions in the aquarium hobby. Not only is it substantially cheaper, but you know exactly what goes into it and can get just about the same accuracy with the right scale.
Something to point out, is that the "Potassion" bottle does have both potassium chloride (KCl) and potassium sulfate (K2SO4) in it. Probably to help keep those chloride and sulfate ions in balance. There is a similar topic about magnesium in the general discussion.