This is something I first noticed in users who posted spam. They would make a post, I would click their profile, and it would come up (unregistered). At first I thought it was their account being deleted by moderators, but I'm beginning to notice it in other non-spamming first time users as well. Is this a result of people not having to make an account to ask a question, or do these people really not want a Chem.SE account?
'Unregistered' means they're not a user yet.
For people with first posts, there's this option called 'post as guest'. They can post a question without having to log in, and their account is only identified by a browser cookie. Accounts made this way are unregistered accounts. There are some problems with this method, but it makes posting much, much easier for new users.
There's a meta.SE proposal to get rid of it, without any official results.