There's a new matchmaking system out in 3.0.8 for arenas, and the change (surprise surprise) did not make it into the patch notes.
Update: Please see this post for further information.
The system is thus:
There is a rating, one that you cannot see, which will match you "against others of equal skill." This rating is different for each of the arena brackets. At the end of each match, the internal rating, the one you cannot see, is changed. The cycle then repeats itself when you queue up for your next match.
There isn't any more information that that, which was released by Bornakk late last night.
Bornakk warns that you might notice some odd behavior in arena matches.