Unpredictable ranks

So.....I'm just doing my normal routine playing a few games of ARAM to warm up before I try and play a game of ranked(because I'm terrible). I play as and do decent. I end up going 16/10/19 and was pleasantly surprised by getting a S- rank. I decide "hey why not play another game of ARAM since the last one went so well?" I end up getting and go 14/1/17.....to get an A rank and a not so pleasant surprise. How is that possible? I know ranks are judged by how well other people do with that champ but is Swain like the God of ARAM or something? Nelson does not know this thing and it is unsettling.