As for win/loss streaks - there are no "hidden" parameters in the matchmaker for win possibilities, it's just plain luck. Think about it, why would we program the system to make people lose time and time again when they eventually just quit playing?
The first 2 tiers are normally isolated by a sandbox for newcomers, but it's designed so that new players get their first impressions and progress to higher tiers where they meet other experienced people. But there's a situation when a lot of players actually stay on low tiers because that's where all the bambies are and fighting is much easier.