X-Plane 9 had a very long beta, and the end of that beta was mostly spent with a finished sim and me trying to fix the pixel shaders for five thousand flavors of video card, driver, and operating system. During this time, Austin started work on new systems modeling features for professional level sims. We branched the code, with my work going into 900 and his going into 910.
When he finished his systems code and I got the pixel shaders fixed and both were fixed, the two were combined into what became 920.
So that's how we "lost" the 910 version number - some professional customers have the version number, but everyone got the features.