I don't know about others, but I rate skill as "How is it worked with character archetype and raw stat" Nerine while has exact same skill, got higher grade from me since her high atk help her fitting in sweeper role while Ōonibasu got less with lower atk. So Ōonibasu in sweeper role got overshadowed by the other girls in the same archetype. While girls in counter archetype will get the basic grade which is B- or B (2.8x with 37% chance will get higher grade by default) since it pretty much in the middle and their main role is to counter so the value stat would be HP and def rather than atk.
Hope this cleared!
PS: it's 1 week old comment but I see you said something like 'Let just skip Saffron by giving her S-tier and moving on?'
Well... OVER MY DEAD BODY my friend! You can go ahead and skip any girl you want but I will not allow my waifu to be left out just like that!