Fix doesn't "randomly sort out of nowhere." It always moves people as necessary so that the teams are ALWAYS even, with only the occasional difference of one.
as such, say you have a 5v5 goin'. somone leaves, so it becomes 4v5. nothing happens. however, ANOTHER person of the same team leaves, so it moves a randomly selected person from the other team onto that team so you have 4v4, and it's fair again.
i don't recommend using it because people always complain because they're stupid.