LIVE mode matches players for multiplayer games based on their LIVE Tier. In addition to this, the game server ‘ping’ is taken into account such that the game experience is acceptable for both players. Finally we consider each player's previous matches to avoid matching with the same player repeatedly.
Things that are not considered during matchmaking are: Team Rating, Win Rate or Streak, any sort of ELO/Ranking/XP score, anything related to player spending, the position within the Tier or Leaderboard
This means that in general players should match with another player in the same Tier, but it is possible to match with a player from a different Tier, but this would only be due to lack of suitable available players from the same Tier online at the matching time.There is no control during matching to manage the difficulty experience of individual players, but as players move between tiers it is expected that opponents will be more or less challenging as a result.