It's possible that some plugins from these vendors have added DLL metadata, where others haven't.
Not a stupid question at all. The way Cubase is able to produce that list is by instantiating each of your VST plugins one by one on first launch (a lengthy and potentially unstable process), and then storing a cached list of the VST metadata (which can only be accessed at runtime, as opposed to the DLL metadata, which can be readily accessed by the file system.) Cubase makes note whenever files in the plugin directory are changed, and then selectively re-scans only the changed/added plugins. That's a great approach for a DAW that actually needs to run the VST plugins, but it does introduce speed and stability problems, particularly for the use case of PluginUpdate (which implies you'd be updating plugins more often than you may have been previously.)
For PluginUpdate to adopt this approach, it would have to be a VST host itself, which would mean you would have to configure audio settings upon first launch, and then deal with the aforementioned slowness and instability during the initial scan.
Rather than take that approach, PluginUpdate for Windows takes the same approach as the Mac version - relying on the metadata supplied for the plugin files themselves. On the Mac, the compliance rate for file level metadata with plugins is over 99%. On Windows, it appears that far more plugins are not DLL metadata compliant, even some popular and highly regarded ones.
So, if you've noticed, the plugin scan in PluginUpdate is incredibly fast and stable. We'd like to keep it that way, but if it proves an uphill battle to get plugin developers to provide DLL metadata, then we may have to employ the VST host method, which ideally should be avoided if at all possible.
EDIT - of course, we could employ a hybrid approach, scanning ONLY the plugins in the "unknown" list as VSTs, which would mean roughly 70% of plugins could be scanned quickly still. Hmmmmm....