If you answer someone else's question and it's accepted, you'd normally get an upvote as well and unsung hero is a consolation for providing a helpful answer that nobody upvoted. If you've answered your own question, there's no such guarantee that the answer is helpful.
Although it’s fine to ask and answer
your own question — this is
specifically encouraged in the faq —
you’ll have to rely on the community
to upvote your answer and validate it
as correct. You, sir (or madam), are
biased. Of course your answer to your
own question will be the best possible
answer. You wrote the darn thing!
While that blog entry is out-of-date and not about exactly the same topic, it still explains why having a self-accepted answer doesn't mean much by itself.