Levels 91 and 92 are "prestige" levels. They are different from the first 90 levels of the game.
From this thread: "We’re considering all the adventure levels past 90 as “Prestige Levels.” Each level is going to take more time than you are used to in order to complete and you’re going to have to meet minimum AA level requirements in order to start earning experience past 90, but you’re going to earn Prestige Points at every 20% (Every other “mini-ding”)."
Also: "The AA requirement won't be 320 for sure (We don't want to require AoD), the bar for earning XP in 90 is going to be 280, because you have your heroic endlines at that point (and I hate large primes so 277 wasn't going to work). The requirement exists so we (the developers) and you (the players) have a little more information about what a level 91+ player is. Right now if you form a pick up group, you really don't have an idea whether your tank has 65 AAs or 317 AAs. The same goes for us, content for a level 90 with few AAs is pretty different than content for a level 90 with a whole lot of them."
That is their reasoning. The requirement makes sense to me. The heroic tree endlines are an important part of a character's development, but it's more than just the heroic endlines. By the time you have enough points to get those endlines, you have helped your character out tremendously with all of the other AA's you have spent. When looked at individually, the points spent may not seem like much, but when you look at the whole picture, it makes a big difference.
I have a templar that was originally raised as strictly a tradeskiller. I decided to start adventuring with her, but she is behind on AA's. I have been working on that with her now and then, but I will not take her to Skyshrine at this time because she would just die repeatedly. I'm not saying that AA's alone will make the difference. It's a combination of AA's, gear, adornments, and knowledge of how to play my character, but AA's DO make a huge difference in her ability and survivability. (Yes, I know HOW to play a templar. My first character was a templar and I played and raided with her for years before I betrayed her to inquisitor.) 