Keep in mind that "flagging" is not handled by a small core set of moderators. The ability to 'flag' a story is granted to anyone with more than X karma (right

Keep in mind that "flagging" is not handled by a small core set of moderators. The ability to 'flag' a story is granted to anyone with more than X karma (right now I can't find any reference giving the exact X value). So whether a story is flagged is based on exactly which random users happen to see it when it gets posted. If you get one random set, they may be more likely to flag it, if you get a different random set, they may be more likely to not flag it. So there's no way to answer the question "buy why did that other story not get flagged" because the reason why is: it depends on exactly which random sub-group of users with 'flag rights' saw the story when it was first posted.