So they're moving up the Chrome OS release pace to match Chrome itself. Really, the only surprise is that they didn't do that right away (hence having to make a slight correction to get them back in sync).
[aboutchromebooks](https://www.aboutchromebooks.com/news/lacros-vs-chrome-and-c...) has a nice explanation as well.
The reasoning I've read behind it is this will allow updates to continue for the browser much longer (probably more important!) than the cycle of the OS which has a much more finite support window.