Honestly, I'm not very familiar with Managed Software Center (the replacement for what used to be Managed Software Update). It seems when you select an optional install, it kicks off the download/install immediately in the background. It appears to me that the Updates tab is merely a UI for that background work, but clicking it doesn't kickstart any processes from what I can tell.
So, you're seeing what I described for most packages, but *must* click Updates to kick off installs for any packages where PackageCompleteURL is specified?