Admins don’t always choose to purge all posts by a user who has been banned, sometimes they’ll just ban the person and remove the infringing posts since if the person was a very active user purging them could take a chunk out of the content on the site, especially on smaller instances, and assuming they have mostly legitimate posts it’s probably the best option to remove specific troll posts after banning than to purge their whole profile.

Also if someone is temporarily banned they almost never purge those people since it’s a temporary ban.