Don't compare your score to your teammates. It's a red-herring, as is KDA. The grading is based on other players with the same role with the same champion, so that 1-9-2 ally doesn't mean anything to what grade you get. Try comparing a support's scores (KDA, wards placed, damage taken, and dealt) to an ADC's scores and try to say who was better, because that is extremely tough to do, isn't it? Obviously the ADC will probably have a better KDA, but that's not really a support's job. What you're doing by looking at your allies' KDAs is exactly that, you're comparing apples to oranges when you're trying to figure out why you got cruddy grapes, it doesn't work.