I have set the settings on Before a comment appears, to Comment author must fill out name and e-mail
and Comment author must have a previously approved comment. I dechecked An administrator must always approve the comment. Problem is that some of the commenters, who do have approved comments, still need to be approved. Why is this?
Did they have links in their comments, and if so, how many? Under settings > discussion in the “comment moderation” section, you can set the number of links someone can put in a comment before it automatically goes to moderation. By default it is set to two.