Riot doesn't have a freakin' clue about what an assassin really is on this game and how to properly score them for their mastery.
Diana's a fighter/skirmisher type. She's lethal as hell, but she has no escape and can sustain herself decently in a fight while most assassins do not stick around, they get in and out. I've heard people say Xin is an assassin, I'd go with broken fighter instead since he's meant again for sustained combat.
Generally if you can stay in a fight and put somebody down, that's a fighter. If your main goal is kill quickly, avoid a struggle, and escape quickly that's an assassin. Of course if you don't farm heavily and push turrets you won't get a good grade since that's what all assassins are apparently supposed to do. 20-0 Akali in 23 minutes gets no S because she didn't have time to get 300 farm? 30-3 Kat? This is troublesome to say the least since that's the only job of an assassin: Kill.