Well, as the title says: is there any logic at all in the teammatching of Splatoon? Is it based on previous rounds, armor, weapon, level? Of course in ranked mode you'll be put with 7 people that have roughly your rank, but what decides in what team we play?
According to Jintor (who translated this site) the matchmaking is based on your playstyle. You are matched against people that have the similar playstyle as you have. This playstyle is determined by the kills/deatchs and area you have painted.
According to the explanation, "regular match" doesn't take into account the closeness of your ranks, but rather the similarity of one's playstyle. Rank is a measure of the amount of time that you've played; in direct matching its influence is negligible.
In other words, people who focus only on improving their kill count will largely be matched against other people like them. In short, kill count, death counts and paint points are the three criteria by which matchmaking is accurately assessed.
Initially, we wanted to make ranks vaguely map to the match-making criteria (?) but gradually began to feel that it just wasn't working. At last, we solved the mystery.
Of course it's not a perfect system, so once in a while players might notice and think "I've been ranked here?", but hopefully players will come to the realisation that they've been assuredly put into the same category as like-minded players (lol).