That's basically what I asked before, I don't think there's a main reason why this is, but I'm not the person to ask...
Again, read the previous posts - sounds like Arn and Mr. Anderson are going to be secretive and tight-lipped about the next rankings, and what post count is required to acheive them - which I think is cool and bastardly, so I applaud them for that.
10,000 posts sounds like a good number for another ranking to appear, but we'll just have to wait and see!
As for what rankings would occur after 601, the 603 would be nthe next logical choice (assuming the 601+ doesn't count!) After that, (anyone correct me if I'm wrong here) there was the 603e/ev, the 604/604e, and then we would start getting into the 7400 series of processors (those were the first PowerMac G4s if I remember correctly...)