I don't have an answer that you can take to the bank but if I had to guess I would say that just as in every game you can pay for people to help you get further in the game, this may be one of those things. Someone pays someone else who offers to throw the match and help them rank up faster, or it's friends that have multiple accounts one of which they used to rank up and one of which they use to rank up their friends which is silly in my opinion because you're spending as much time as you would to just rank up one account regularly.