[CLIENT] Season's highest grade is not always accurate

So I confirmed something I had long suspected: The "season's highest grade" values shown in the client are not always accurate. I noticed today that it claimed my highest grade on Brand was an A, but I knew I'd had a recent game where I got an S+. Sure enough, that game was still in my match history on the client even.

Only reason I can think of to explain this is that I probably wasn't eligible for a box when I got the S+, but I have some champs where I have yet to get a box and have a highest grade that is S- or better, so it's at the very least inconsistent.

Evidence: S+ Brand match: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2512500854/35110458?tab=overview A on Brand for season's highest grade (open full size to see): http://imgur.com/a/ZQTof