I own Moderated group but posts appear mysteriously Mod activity shows "approved by System" How?

Some members seem to be able to approve messages who are not moderators. How? Additionally some messages seem to be approved by no one at all it just says "the system". I know by checking Moderator Activity. I would not have approved most of the messages getting through. This particularly could be a problem with Yahoo turning groups into "Adult" category rather arbitrarily (thus losing search-ability in a group search). Thanks