So, you played a ranked game. You totally destroyed that Jax, so much that he won't be able to recover before you end the game. You are hard pushing toplane. Game ends. You don't get an S. Why did this happen? Well, you did get 7 kills, but enemy jg killed you 3 times and you didn't help your team at all. You didn't help with Herald or Nashor, and you totally didn't assist them. You didn't ward at all, you didn't place Pink Wards. So yeah, that's why you got an A-. To get an S, you must help your team with turrets, epic monsters, kills, and lastly, have good vision points. Regardless of your role, if you do all of the things mentioned, you'll get an S. It's not just about your KDA, you have to help your team as much as you can.