I don't know, but something I'm sure of, they have a different (more advanced) build in house than what we have now. Its been this way since the beggining.
THIS
I also think they currently have some kind of (currently unfinshed?) beta 4 which will be released on 22nd as the "preview build" (or gamma) which is the release candidate. So I'm sure there will be more features that go directly into gamma. They had 2 weeks since the start of beta 3 and still have 1-2 weeks to put additional features in for gamma release.