alt Hacker NewsSpeech writers and shadow writers in general are writing on behalf of other people, but they're not writing the same thing. They can't! Usually they're writing something better, with the proper voice and authentic to the views of the person, but still not the same as what they would have written.
Depends on the individual. It’s usually a highly collaborative process. But in general, you’re correct that you’d expect a good writer to write better than the person who hired them.