Actually they have given you everything you need to know about how the ranks are figured.
You are graded on your overall performance of the champ you use in the role you are playing against others in your region using that same champ in that same role. There are a lot of factors that are included in determining your grade which include your kills, deaths, assists, towers taken, wards placed, wards destroyed, dragons taken, baron, movement around the map and your cs.
Sadly the system does not account for game length. And from the looks of it, it looks like either your team just smashed them quickly or they surrendered. You was preforming well bu tthe game just didnt go long enough for you to do what was needed to get the S.
Heres a tip........get a duo partner. You 2 que up for bot and support. Except you que up for support and they que for adc. But you play Miss Fortune and they play whatever support they want. You will be graded against anyone else that played Miss Fortune as support (which shouldnt be many people). Just dont feed and get about 4 or 5 kills, place a few wards, and help with objectives and most likely you will get a S.
its a flaw in the system, use it until they fix it.