Is there a way to know which are the reasons for a flag to be marked as invalid or an edit rejected?
-- EDIT --
My decision for this issue is that, given that I can't get an explanation, I'll express my opinion anyway with a downvote.
The short answer is because it was not what you thought it was. If you flag as "not an answer" and it is marked as invalid, then it really was an answer.
When I've asked about this kind of thing in the past I've been told that if you are a bad flagger the mods will email you. Or perhaps just show up in your home when you least expect it. Probably the latter.