How many total kills were there in that game? For a 50 minute long game, if you only get like 55-60% of the kill participation as a JANNA
(This bitch is an assist whore) if you get lower than 75%, you are probably adequate compare to the other Janna. This system does taken into accounts to how well the other support Jannas in your mmr do. This way they don't compare something like Leona and Soraka together, 20 minute surrender and 1 hour long are caculated differently and Annie support and Annie mid won't get mix up. A champion like Janna that excel in getting assists should have well above 24 kill participation in a 41 kills match.
KDA is also not the only thing that is taken into consideration, many has said that objectives and wards are also a part of this grading system, for the support at least. I don't believe they count damage dealt into consideration when it come to support, if they do, then I have no clue on how that work on a role that have no damage itemization.