If you look at your extended_states file, there's likely to be a few packages which have Auto-Installed: 0. These are manually installed, right? That's what apt-mark showmanual thinks anyway. Why are these there? If the intent is to only list packages which are automatically installed in extended_states, then you should remove that field and just make it a list of automatically installed packages. Don't see why there should be two different ways to indicate if a package is manually installed.
However, I don't think you should convert extended_states into a simple list of packages. There's a widespread impression that extended_states shows manually installed packages, as in
things that the user really wanted to install (and not dependences like foo-data or system installed; exceptions like these should be in the man page and are completely counterintuitive). Right now apt-mark showmanual shows probably hundreds of things I didn't take the time to install. I'm not the only one who notices that: see the
comments to accepted answer on the page I referred to previously (on AskUbuntu actually) What's the reason for that?
I'm getting the impression that the manually installed metadata is actually designed more for package maintenance and maintainers than for users who want to get a good idea of what they've done in the past. However, there's tremendous interest from users in getting a simple list of what they've installed. It's a reasonable request. Right now there's no simple way to do it. People are asking on the Q&A sites and getting responses which show horrendous and clunky scripts that don't even work or require you to hunt down the manifest of the distro's packages. It actually seems like the current set-up leaves an elegant opening for this: if something has the Auto-Installed: 0 in extended_states, it could be indicated as truly auto-installed, while the ones which aren't even listed in extended_states as auto-installed could be pseudo-manually installed (system installed plus things like foo-data).
See
1,
2,
3,
4,
5. Going back a little further, there's
a 2008 thread on
ubuntuforums.com.
I suppose you could switch this to a wish, although I still think there are bugs in the documentation and how this is precisely working.