With Chromebooks and the Chrome OS still being relatively new I do not think that Google always has a clear plan on updates. As they find new bugs they may change the update schedule. I think that is why they don't announce them.
Yeah, the Chromebook does get automatic updates. It is weird because you can also click the wrench icons to check for updates. Sometimes when you press it there will be updates that didn't get automatically updated like it is supposed to.