My comment on the question seems to be not quite right
Instead there is this Meta.SE post about why a flag can be marked as declined but the comment or post is still deleted/edited/closed:
Why does flag marking as helpful/declined not always correlate with moderator action?
A number of things can happen:
- A mod might not agree with a close flag but 5 others might come along and vote to close
- A mod might disagree with your flag reason, but still agree that the post or comment has issues and deal with it - it's important to flag correctly
- 3 regular reviewers choosing do not close will also decline your flag, but again, others might come along later (or a mod) and close the question
- A mod might click the wrong button when handling your flag - I've seen multiple questions on meta.SE about this. One or two accidentally declined flags aren't a big deal.
A mod can only accept or reject all the flags on a post at once, and if the mod doesn't want some of the flags to apply (e.g. posts marked and accepted as spam carry a penalty), they have to decline them all.
(I still think I've seen something like my comment on Meta.SE, but can't find it now.)