Yeah the weightings are probably different, and I'm sure it's not truly perfect (and even knowing what 'perfect' play is in the first place is dubiously possible), so occasionally people taking cs/kills when they shouldn't might be interpreted by the grading as securing cs/kills while the carries are backing or something, but it's all a big feedback loop.
To expand on that, if a support is taking cs/kills, then the carry won't get as fed, which will mean the game overall might go poorly, which means the support will probably be stuck in a position where they can't get kills/assists since the enemy carries are comparatively fed, and so in the long run their score ends up worse compared to if they played like a 'good' support.
If the support can ks/cs and still have the game snowball, then I expect they can get away with it and it'll seem like a good performance rating wise (and I mean, it was if the game was that snowbally), despite that not generally being viewed as appropriate support behavior.