Here's how grading is calculated, not only, but all of this information is readily available to everyone who plays, since it directly comes from the client:
If you click into any given champion in the 'STATS' tab in your profile, you'll see more in depth of how they calculate any said grade.
In Combat, you get graded by: relative KDA, relative kill participation, relative utility score, relative damage per death and relative damage share.
In Income, you get graded by: relative damage per gold, relative early gold advantage, relative early CS advantage and relative CS per minute.
In Map Control, you get graded by: relative objective control, relative vision score per hour, relative roam dominance score and relative kill conversion ratio.
That means, your overall grade is an average of 13 different stats. You only mentioned 16/0/5 with 130 farm at 25 minutes, which are only 2 generalized stats out of these 13. Sure, 16/0/5 is a great score with a perfect KDA, good work on that for sure, but that only scores you highly on the Combat and a few in income and map control. Your CS of 130 at 25 minutes isn't bad, but it really doesn't reach an S+ grade. Someone mentioned that in 25 minutes you only placed 6 wards, meaning you probably got only a B+ in map control at best (for that given game).
So all things considered from what you've (and others have) given, you could've gotten an S+ in both Combat and Income, but a B+ or even an A- in Map Control would drop the average overall grade from S+ to S-.
So honestly, your score is quite justified.
Any further questions?