I'm building a country level search engine at the moment and one of the biggest quality issues is compromised sites. Many of these sites are using Joomla and out of date plug-ins. It is probably easier for Google to e-mail people about the problem.
@BillyS Joomla has a particular page structure/site architecture that differentiates it from other CMS. Perhaps Google was just advising all people using Joomla to upgrade. Then again, I've given up trying to figure out what those twiddlers in Google are up to as it is just too much unnecessary stress.