They haven't told us exactly what the requirements are to get it, but what they have told us is it is based a lot on how people with the highest win rates and play rates on them do to win games. They have also said that they balance stuff for individual champions as well. For instance on an adc cs is probably balanced very high, but on a support like janna, it probably has minimal impact. If I had to guess, I would say it probably has to do with objectives(like drag or towers), and maybe a bit of kill participation. Also zed would probably be highly balanced on kills and deaths, so it could also be possible that you were 1-2 kills/deaths away from an S-, same with all the other champions you mentioned. They should all be balanced on kills and deaths a lot. There might also be some enemy jungle cs requirements for rengar and kha'zix.