It seems unlikely that players are divided into specific divisions in ranked. Instead, I would assume matchmaking is based off of the numeric value shown when choosing categories in the "find match" menu. My guess is the value is averaged out over the team, them the matchmaking system tries to find another team close to that value.
So a 2v2 team with [100, 750] could be matched with one with [350, 500], since they both average out to 425.
I would assume unranked works in a similar fashion, just with a hidden numeric ranking that changes more rapidly with wins and losses.
Basing the matchmaking only on the "Player Level" (tied to the Rookie, Semi-Pro and Pro titles) seems very unfair since it can be artificially inflated (by playing against crappy rookie bots in solo) and never goes down.
Thus the game would assume that playing more matches automatically makes you better, even if you lose 90% of those matches.
It seems much more likely that there's some hidden rating that increases when you win and decreases when you lose (and the matchmaking would be similar to that in ranked, just not visible).
A matchmaking purely based on statistics doesn't really make sense either. A ranking system has to be responsive and reflect a player's current skill, not the average skill since first playing.
However, this is just speculation based on how it appears to work and experience from other games. We can't really say for sure unless Psyonix tells us about the inner workings of matchmaking (or someone does some extensive amount of science).