In a DNN 4.0.2 install that is set up so that users can register themselves without the registration having to be verified by email or by an administrator, I have noticed some users in the user list in the Authorized column have a darker gray box with no check mark in it. 

What does this indicate?

Registration is set to public, althought I went the long way around of saying it by saying that it was not set up to require email authorization or validation and that the admin does not have to authorize the account. 

Seems to be sort of hit-and-miss on the users as the majority of them have the Authorize box checked, it is the minority of them that have the darker gray colored box without it being checked.  So I was curious as to what might be causing that type of effect when the registration is public.