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Flagged answer was “helpful” but was not deleted

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

An answer I flagged today as being of a "very low quality" was deemed to be "helpful":

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yet it was not removed. Note that I don't mind it still being there (the question is closed, so it might even get deleted with the question in due time), but I thought that "helpful" flags always caused the answer to be deleted. Am I wrong, or did something go wrong in this case?

I scrolled through ~10 of my "very low quality" flags that were "helpful", and all of them were removed by a mod.