18/5/16 Evelynn and an A+...

Wards, minions, towers, etc. all come into play with the grade. K/D/A, while a large factor indeed, is not the only thing taken into consideration.

Not to mention that if you went 18/5/16, meaning you had a kill participation of 34, your team could have had a total kills of 70 or something.

Your grade is partially tied in with your team's performance, and I don't mean winning, but overall kills and your participation %, I think you need to have something like 80% kill participation, paired with other factors as well in order to get into the S rankings. It may actually be higher.