New Post: Version check through registry keys. I don't really understand...

First, I must admit that I'm not a Wsus expert.
Wsus is a pretty much well documented role (for a Microsoft product :-) )

This is what we can learn from MSDN :

• There are three core applicability rules, IsInstalled, IsSuperseded, and IsInstallable. These applicability rules appear under the InstallableItem.ApplicabilityRules element under the SoftwareDistributionPackage element (Update Level). The IsInstallable and IsInstalled rules can also be specified at the top level of the SoftwareDistributionPackage element (Package Level). If they are specified in both places, the expressions are combined (using the Boolean AND operator) to form a composite applicability rule for the package.
  

I have read somewhere an explication for why there are two type of rules (update and package level).
At first, when Microsoft started to think about Wsus, they have imagined that a package can contains more than one update (InstallableItem). So, they have designed a package-level rule and an Update-Level rule. Each InstallableItem should have his own update-Level rule.
Imagines a package to install Office. One InstallableItem contains the main file for Office, a second InstallableItem contains a patch to apply if Windows is installed in French, another InstallableItem contains the same patch for the English version and so on....