Don't compare your own champion and role performance to a different champion and a different role performance on your own team, thats not how mastery grading works.
You don't gain grades in comparison to your teammates, you gain them in comparison to people in your region who play the same champion in the same role as you.
Apparently that Yorik did very well, compared to other people playing Yorik in the support role. You did well, but not well enough to gain a S rank when compared to other Viktors in whatever role you played him.
Also, you can't compare certain stats between all roles, you can't expect a support to get a lot of cs or kills, so you can't really compare those stats with other laners.