|You can add the users to the site in two ways. You can add the users explicitly like domain\username or set NT AUTHORITY\authenticated users. So, directly added users can access the page and also the users in the Active directory.
When an user from Active directory browse the site, the user get added to the AllUsers group.
Web.Users: Will list all the users who are added explicitly to a group like domain\username.
Web.AllUsers: contains any user that has accessed the site (both explicitly added users and users who is in the Active directory accessed the site once).