Is there any reason why this happened? Is this due to a bug?
Moderators can only delete, dismiss, or edit a comment. We can't even add a reason if it was helpful or unhelpful.
While you can retract (and reflag) posts, you cannot do that for comments. So in the case of comments, I might disagree with the flag, as a whole, but agree that the comment should be deleted.
The reason I decline in such cases is to signal the user "this is not the appropriate flag". Hopefully, they will notice that the comment was indeed deleted, and infer that the idea of flagging it was correct, but the execution faltered.
Yes, this method sucks. But it's the best we have at the moment. I'm all ears to suggestions on how to improve it.