I saw this accepted answer that was at the bottom of the list:
The answer was posted by the same person as the question poster and the order was set to "votes". Is this usual behavior?
Accepting an answer is an easy way to add extra visibility for a particularly useful answer.
If that would work on one's own answers, the feature would be tempting to be abused for self-promotion - highlighting one's own answer as "useful" although it might not actually deserve it. This would work against credibility of the site. So an answer accepted by author will have to earn its special place the hard way - by votes of community.