Howdy,
That is because 4.2.8 has just been pushed to auto-update. We usually wait a few days between the release of a new version and pushing it to auto-update (and thus pushing it via email). This allows time for any last minute bugs that appear in the new release to be addressed. Generally, these are few and inconsequential in nature, but, if you wish to be extra careful, you can wait for a release to be pushed to auto-update before upgrading.
Cheers,
Michael