Is there something behind Alyson Stoner's appearance in only odd numbered sequels of step up films?

I'm going on speculation here, but I think it's more chance than anything else. Around the time of Step Up 2, she was busy starring (in a better role than the one she had in Step Up) in Camp Rock. This is an interview with her:

Another movie you have coming out this year is Step Up 3-D. What was it like working on that?

I loved it! I felt like such a newbie compared to all of the other dancers who have been dancing longer than I’ve been alive. Jon Chu is a visionary. He has a great eye for what reads best on camera and the fact that the movie is in 3-D technology just takes it to another level. Everyone involved is really passionate about what they do so that translates on camera and we’re hoping that people just jump at it like we have. It’s been our little baby. We’ve been working on it forever now so we’re excited. It comes out this summer. You’ll have to check it out

You were part of the first Step Up. Was it fun getting to return for another chapter in the series?

Definitely. I had watched a portion of Step Up 2 and actually believed that the lead girl was an older version of my character just with a different name. I didn’t know if they were trying to make me older. Then when I heard from Jon Chu that it was a totally different storyline, my hope was restored and it gave me the chance to work on this one and oh boy, I just took it and ran with it. It’s something I’ve never done before so it was very new and a bit of a challenge to learn all the dancing, but I had it easy compared to the other dancers. You should see what’s going on. It’s absolutely mind boggling. But it was really fun. We really had a great time.

Firstly, the italicized portion strongly suggests it was her own choice not to star in the film, as opposed to the director/casting team. Secondly, it doesn't seem to suggest there is any bizarre reason behind this. It seems very logical that she simply had other projects to do and wanted to do them, but was excited at the chance to go back for Step Up 3.

As for Step Up 4, I'm not sure of the exact reason. I've read that she started college at the time of filming and her schedule couldn't handle both. I also know she had her heart set on auditioning for the role of Katniss in The Hunger Games. Perhaps disappointment from not getting that role turned her away from filming for a short while?

Obviously there's a lot of speculation here, but I just wanted to point out that there doesn't seem to be any superstitious or otherwise bizarre reason for her appearances in only odd-numbered Step Up films. It just seems to be chance based on her life situation at the time.