“Tested up to” version is different than the one on wordpress.org

That is because some plugin developers do not include a full version number in the readme.txt file of their plugin. WP Plugin Helper gets that information straight from the readme.txt file and not from wordpress.org website. If the difference is only in the last number of the version (for example 4.9 on local site and 4.9.6 on wordpress.org) there should be no reason to worry as the change should be backwards compatible. For more information about semantic versioning check this link.