Unlike every other desktop operating system, Chrome OS updates are rolled out on a per-device basis, instead of being pushed to every Chromebook at once. This usually results in a less-buggy experience, but it can also mean updates for certain models become delayed.
If you’re trying to figure out what versions of Chrome OS are rolled out to which devices, there’s now a site for that – cros-updates-serving.appspot.com. Not the most memorable name, but the site shows the update rollout status for every supported Chromebook. It also provides links to the recovery images for every model on every Chrome OS branch. Handy!