After looking a little further I found an article on the Webmaster forums page. It has nothing to do with your hosting provider as I earlier suggested. It is 100% Google. The reason is to assist webmasters in protecting their sites by keeping up to date services.
They need to make a few improvements since each time I've received a notification it has been about 1 month after I already upgraded. I guess it depends on how often they crawl your site. This is when they discover the version you are using. Another reason they might get it wrong is because I actually exclude the WP version as suggested by many, as a security measure. This could explain why they send me notices, since they don't know what version they must send a notice as standard procedure.
Here is the link to the Official Google Webmaster Blog where it explains the service: