Question about end game rating

You know when you finish your game, click on that continue button and get that big ole letter in the top right corner of your screen somehow indicating you on how well you performed. After looking at some screens (mostly on fb groups) I've wondered how these were calculated, some people with perfect KDA and a lot of objective involvement just couldn't get S+ while people with poorer KDA/creep score could achieve it. Then, some people went on a 5 man pre made mission to get the lowest grade possible, and similar things happened. Could someone please enlighten us on how the system works ?