With the release of patch 3.0.8 we have implemented a new matchmaking system for arenas that will improve the overall system and make sure that players will be matched against others of equal skill. The new matchmaking system makes use of a rating that you can't see and is separate for each of the arena brackets.
At the end of each match, the rating changes are affected by this new system and as it just came out, there may initially be some odd behavior in rating changes but these will straighten out fairly quickly.
We will monitor this new system and update it as needed to help provide the best experience possible in arena matches.
Above all, the primary goal of the new Arena matchmaking system is to match players against opponents of similar skill, regardless of your or your opponents' team rating. This means that once an estimated skill level is determined, you will likely be matched against other players of a similar skill level. Note that the estimated skill level is constantly being evaluated and will adjust based on each team's continual performance. Ratings that are won or lost take into account skill level, so that you won't suffer a high penalty if you lose against a team that is much better than their actual team rating.
Another important aspect of it is a smaller range of maximum rating that can be won or lost. Because of the differences between how the new system assigns rating points at the end of a match, we are still tweaking rating gain versus loss per match to improve the experience.
If you were previously ranked with a high skill level and decide to create a new team, it should be much easier to reach your team's correct rating. This ensures that players won't feel penalized for moving between teams and high ranked teams won't feel penalized for trying out new players without much risk to the team rating itself.
To join in Arena games is the core and fundamental reason for the new Arena matchmaking system for player which is very fun. We feel that with this new system, higher ranked players will most likely find the challenges that they seek, newer players will no longer feel that Arenas are closed to them, and players will not feel as penalized for trying new things in the Arenas.
Because of the way the new system works, there has been some question as to the purpose of each of the different ratings.
Team rating is the primary measure of a team's skill during the course of its existence. Titles and arena points are based on team rating.
Personal rating only exists to ensure that a player has participated enough on a team to deserve the benefits of being on that team. When personal rating gets close enough to team rating, we judge it is likely the player deserves the benefits of the team.
Matchmaking rating exists to ensure quality match-making despite players changing teams. In addition, the theoretical mathematical foundations of the new matchmaking system allows this rating to adapt much faster than team rating and personal rating, making the entire system more correct. In our measurements to date, this rating has been quite precise in predicting the outcomes of future matches.
   At length we like the way matchmaking works in the new system and feel that it is working as expected. However, we acknowledge that there are some aspects of the way rating is calculated under the new system that could use some tweaking. As with all things, your enjoyment of the game and its systems are very important for us and this system was implemented in an effort to further promote the enjoyment of the Arenas. As always, we welcome your feedback and would constantly be striving to make this system even better.