mastery grades make no sense?

They're not supposed to make sense, they're supposed to measure unknown metrics which prevents padding them.

"It’s based on the champion and role that you just played in a game. So, for example, we look at Annie mid differently to Annie support. We take a series of performance metrics and compare how you do to all other players in your region in your champion / position combination. It is percentile driven - so earning a high score means you performed in the top x% of players on that champ in that position. The grade itself governs the point gains."

• Taken straight from Riot's announcement.

Because the system thinks the 0/0/27 is a better kda

It also counts gold gained (which you had about 2k more of in the S over the S-

What makes no sense is that I have gotten similar to both of those stat lines and not even gotten S

Or I get identical stat lines, with maaaaybe a 200-300 gold different on two different support champs and one got an S+ and the other got an A-

it makes no damn sense at all. There are "rules" to the scoring, but they seem to change every game, every champ, every time you blink 3 times.