im just guessing here.....
the domain users are comming from active directory and the other users that only show first and last are created localy or directly on sharepoint ;)
creating a sharepoint account that is local to sharepoint only:
Add a user account
1.On the Site Administration page, under Users and roles, click Manage users.
2.Click Add a user.
3.In the User area, click Add a new user with the following information, and then type the username and password for the new user
account.
4.In the User Role area, select a user role for the new account.
5.Click Add User.
create an account from an existing account found on active directory that would contain the domain name:
Add an account from an existing network group or domain Use this
method to either add an existing user with a domain account to a web
or subweb, or to add an existing user with a local machine account to
a subweb.
1.On the Site Administration page, under Users and roles, click Manage users.
2.In the User area, click Add user or group name, and then type the account's group or domain name and username, for example
mydomain\username1 .
3.In the User Role area, select a user role for the new account.
4.Click Add User.
http://technet.microsoft.com/en-us/library/cc768246.aspx