I have read a few posts on various boards from distributers of S100/P21S. Apparently at one time, they were in fact identical and P21S was marketed to car enthusiasts at a greater cost because people were willing to pay it. Typical marketing/business decisions. Eventually, flex agents were added to S100 and other than the flex agents, the two waxes remain the same.
That is the story as I have read it. Who knows how truthful it is.