Letter rankings

So, there are too many posts in too many places for a newb like me to figure out what the deal is with the ranks. All of them want to explain what goes into it, how to best get an S rank, but I'm not understanding the ranks themselves. SABCD, right? Why S? why not ABCDE with A as the top? or, better yet, ABCDF like the US grade system, where it's ABCD or Fail? Since I found out there were ranks, I haven't had below a B ... sounds like you'd pretty much have to be greifing to get anything below that?

Now for the rant portion ... I just got out of a map where I got an S. We lost and my teammate went frikkin nuts that I got an S. This was a guy screaming his head off pinging like nuts, to go get the big pink dragon, while we had the two last-resort turrets down, a mostly fed opponent unchallenged in our base, giant minions attacking, and ... to top it off I hadn't even finished respawning after a death! I think, in general, they thought I wasn't helpful. Early on, they accused me of dying , but what was really happening was that the person I was with wasn't gauging when to fall back until it was too late, and I was taking a Base-back when they said it (I was Ashe bot supp, fwiw, and I had low health mana and was in trouble). At another point someone was joking that I was stealing kills from them and they needed gold (it was pretty late in the game, I'm pretty sure they were joking), but I responded seriously that I was just doing a minion push because a big wave had built up on the top and bot. In general, I feel like I contributed, did my job, and the loss could've been avoided. I know it's debated whether the letter rankings are awarded appropriately (no system like that will ever be exact), but in this case, since I'm not a scoreboarder and I wasn't scoreboarding, I think I may've actually deserved my first S, even though I was made to feel bad about it.