In Download Assistant, it’s indicating a great deal of content as having never been installed. Similarly, in Library Manager, it shows what HAS been installed, and as far as I can tell, the information is consistent with what DA shows.
BUT, from within Cubase, all the content that I have supposedly never installed is right there, accessible from the pane on the far right.
WTF? 
Yes, I’ve noted this inconsistency also and it’s annoying. I wonder if the download manager considers something installed if the .pkg file is still present. Which would be stupid because they take a lot of space and should be automatically deleted after installation. And both apps, download assistant and library manager should be aware that the content is (or is not) installed.
As far as I know Steinberg does not really want the Download Assistant to show what is installed. It does not seem programmed to do that. It rather a coincidence if it displays the installed items correctly.
Don’t shoot the messenger, please.