When you have 20 assists as support...

... and you didn't buy any wards, or a sightstone, or the right trinket for fighting the Vayne you were up against, AND half your kit is designed for getting easy assists...

The S- you got from going 0-13-20 and losing the game for your team rightfully gives you the same amount of mastery as when you went 0-20-5 and won.

Just in case anyone is confused as to why an S on a loss doesn't give even half the mastery of a winning B.