A common question we’ve been getting is “How come my third party tool says that a file needs to be updated, but when I go to Microsoft Update, I see ‘There are no new updates available for your computer’?” This is because Office updates utilize Windows Installer technology to determine if a file needs to be updated. When you click “Check for Updates” in the Windows Update app, the Windows Update agent determines which Office updates are applicable to your machine based on the products that are installed. This is done by evaluating applicability rules as defined for each update. To learn more about applicability rules used by the Windows Update agent, review this article: http://msdn.microsoft.com/en-us/library/bb902473(VS.85).aspx
Using Windows Installer to detect applicability has several benefits over file based detection. This includes the ability to update multiple products, repair functionality for missing files and components, and support for reliable uninstallation of updates. To learn more about Windows Installer, review this article: http://support.microsoft.com/kb/310598
Other tools that use file based detection utilize a list of expected file versions when scanning a machine to determine which files need to be updated. This can sometimes be problematic since an Office update may no longer contain a specific file after support for an old baseline (e.g. Service Pack) has been removed, but the file based detection tool still expects the files to be at that specific version. Thus providing you with a false positive result from the scan.
Using Windows Installer to Inventory Products and Patches: http://msdn.microsoft.com/en-us/library/aa369558(v=VS.85).aspx
To learn more about tools that can be used for detection, review this article: http://support.microsoft.com/kb/961747