To the best of my knowledge, Riot doesn't try to predict what you'll pick. I've certainly never seen any posts about that.
The more I play Xerath Mid over any other champions the more my Mid Rank/MMR actually represents my Xerath Rank/MMR. But if Xerath gets banned, or picked by the other team, or I've been on a losing spree with him then I may swap to another champion.
I'd also consider one-tricks (or nearly one-tricks) to be an outlier rather than the norm. Particularly in low elo, it's not uncommon to see someone who's played 100+ matches and their most played champion they've only played 5 Ranked games on. They just bounce around until they find something that kinda works and play it until they get bored or it doesn't work anymore.
All of this is to say that building a matchmaking system off of what you think a person might play is incredibly dangerous.