Why does flag marking as helpful/declined not always correlate with moderator action?

Case 1: your flag was helpful, but no apparent action was taken. Causes include:

• An edit or other action that occurred between your flag and the moderator reviewing it meant that moderator action wasn't required. However the moderator felt you were right at the time of flagging, and so marked your flag helpful to reward/encourage you.

• Your flag was borderline, and the moderator felt it wasn't a bad enough post to act on it, but wanted to let you know you weren't exactly wrong.

• A moderator took an action that isn't visible to you or other normal users, such as sending a moderator message warning or escalating the issue to Stack Exchange staff, in response to your flag.

• Flags to close a question will be dismissed as helpful as soon as another user votes to close it for the same reason you flagged. However, if you don't have enough reputation to cast close votes on the question, it will appear as if nothing has happened until more users vote to close and the question is actually closed.

• Your "not an answer" or "very low quality" flag on an answer was reviewed by normal users who recommended deleting it, which marked your flag "helpful". However, the answer may not be deleted for a number of reasons:

  • The answer was deleted, but the author manually undeleted it. To verify this, check the answer's revision history. If this is the case, and the problems have not been solved, cast a custom moderator flag, as a moderator's deletion will prevent the author from undeleting it again.
  • The answer received a consensus in favor of deletion, but as it had a positive score, it was not automatically deleted. In this case, an automatic moderator flag will be raised. There is no need to raise another moderator flag in this case; unfortunately, there is no way to know if and when a moderator later reviewed the post.

  Once the post has been reviewed with a consensus, you will be able to see a link to the review task in the timeline.

• If the post (question or answer) was edited since you flagged, any "very low quality" flags will be automatically marked helpful, and any "not an answer" flags will also be marked helpful if the edit was made through the Low quality posts review queue.

Case 2: Your flag was declined, but exactly what you wanted to happen happened (e.g. the post was deleted). Causes include:

• A moderator disagreed with you and your flag was declined. Later, other people agreed with you and also flagged, and a second moderator who saw their flags agreed with them and took action.
• The flag type was wrong, but the post still had issues that would cause action to be taken. Because the review queue uses flag metrics for its baseline and the system puts the post into the appropriate queues based on the flag type, it is important that flag types match why action should be taken against a post. Examples include flagging an answer as "very low quality" when it's not entirely unsalvageable, but it makes no attempt to answer the question (i.e., "not an answer"), and using a custom "in need of moderator intervention" flag where another standard flag type should be used (such as closeable questions and blatant non-answers).
• If your flag was a recommend closure flag, if it receives three "Leave Open" responses in the Close Votes queue without a single close vote, it will be marked declined. When you flagged, this is what happened, but later others decided that the question was worthy of closure and therefore closed it.

Alternate explanation that applies to either case: a moderator clicked the wrong button by mistake and it's not a big deal. For genuinely bad posts, other people flag them too, so they end up solved.