Blizzard has once again changed the way arena matches are determined. This is in an effort to make the fights more fairly balanced. The system is apparently implemented as per the note from Blizzard.

With the release of patch 3.0.8 we have implemented a new matchmaking system for arenas that will improve the overall system and help ensure that players will be matched against others of equal skill. The new matchmaking system uses a rating that you cannot see, and is separate for each of the arena brackets.

The rating changes at the end of each match 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.

Source: New Arena Matchmaking System

However, it has caused problems and is Arenas are now temporarily disabled.

We have temporarily disabled all Arena matches while we investigate current rating calculation discrepancies. Further updates will be provided once we have additional information to share.

Source: Arenas Temporarily Disabled