S, A, B etc Ranking Explanation?

Hey all, I'm looking for an explanation towards the ranking system on champions. I don't really understand it much, I'm trying to get Lee Sin to lvl7, played a game, got an S while going 14/9/12 with 25 wards placed & only 78 cs with 2 towers, 21k dmg in 29min. Then i play him again, go 15/3/8 with 22 wards placed & 65 cs with 3 towers, 20.5k dmg in 23min and only got an A. How is that such a drastic difference? And if anything I would think my 2nd game would warrant an S not the first game. Any clarity would be much welcomed :) thank you!

Edit: Both games were wins