Microsoft today released Windows 10 Mobile build 14393.221 to the Release Preview and Slow rings. As of right now, there is no such cumulative update available for PCs.
The last update for Mobile Insiders, build 14393.187, rolled out just 12 days ago. The next day, build 14393.189 rolled out to everyone; in fact, that leaves Mobile with a higher build number than PCs, which is still at 14393.187.
Seeing three cumulative updates for phones before getting a new PC one is virtually unheard of, so as always, this is great news for Windows 10 Mobile users. Since it's so long before Patch Tuesday - which should be two weeks away - it's entirely possible that there will be an update with a higher build number before then.
There is no changelog for the build just yet, which isn't surprising at all. Microsoft very rarely releases a cumulative update - especially for Windows phones - and immediately offers up release notes. If the firm does post something, it will likely show up in the Feedback Hub app.
Source: Windows Central