Your grade is split between how well you do relative to everyone else in the game you're playing, and how you compare to other players playing your same champion in that role.
Your Ryze support game for example is skewed extremely in your favor since virtually nobody plays support Ryze and those who do were probably picking around even more than you.
Thresh on the other hand has a loooooooot of people doing quite well on him. Furthermore, 20-30 assists? It's not hard to imagine your teammates being hyper fed as well, and they get a high grade. As you are getting compared to all Thresh Support players AND all of your teammates and enemies, both were probably just stacking up to put you just below a top performer in that game and as a Thresh Support in general.