Yeah, everything Allison said.
For my first DH race, I raced beginner, because I had never even been to mountain bike race. It wasn't a good fit. So I moved to expert. I still think I won a bit too easily (considering that I had next to no training, and I knew that I was in poor shape). For the next DH race, I went to vet pro, where I simply got a mid-pack finish. And considering my time would have still won the expert class, I probably found where I belong.
So for 4X, knowing that I'm not a beginner level rider, I chose sport for my first race. But for the first couple races, because I was faster than only three other people doesn't make me an expert in that discipline. On top of that, I've watched the experts, and they're in another realm that I can't even compete with, and realistically, I'm not going to get any better or any faster than I am now (despite learning a lot in the last two races). As a result, I stayed in the class for the remainder of this series where I think I belong. I'd also like to earn a jersey.

And with some of the new talent that's showing up in the sport class, I have to give it all I have - and a bit more just to stay on top. But even having said that, I think that my time winning in the class is coming to an end. Those guys are young and fast, and very aggressive, so it's a good thing this series is almost over.