From the Stack Overflow FAQ:
If the bounty was started by the question owner, and the question owner accepts an answer during the bounty period, and the bounty expires without an explicit award – we assume the bounty owner liked the answer they accepted and award it the full bounty amount at the time of bounty expiration.
Why isn't my answer awarded (it has been accepted during the bounty period)?
=== UPDATE ===
Meanwhile, a different answer (half) arwarded.
Should a during the bounty time updated, most (new) votes received and accepted answer don't have more priority than a not accepted new answer with less votes?
I'm very surpised, and think that the bounty starter thinks similarly now. I'm just a little disappointed, because I think many other users also don't understand this priorities of auto awarding.