"unauthorized" is a bit different than "unauthenticated". The former suggests trying to access something you don't have permission for while the latter suggests you're just not logged in.
At a guess, I could imagine some sort of failure of cached pages, which can be cached for signed out users but probably not for signed in users (as the rendered HTML would need to have user context like their avatar etc)
At a guess, I could imagine some sort of failure of cached pages, which can be cached for signed out users but probably not for signed in users (as the rendered HTML would need to have user context like their avatar etc)