The idea is that the very first item you read (regardless of the sort order) should be the accepted answer. I imagine this is intentional by Atwood and company because when someone visits a question from google, they don't want to have to go through the whole "expert sex change" scroll to the bottom to find it problem.
The accepted answer is considered anchored to the question. It is essentially a continuation of what the OP wrote.
When it is 'unanswered' there are different ways of ordering the 'possible answers' depending on how you want the information displayed. But you always want the Question displayed at the top, and the accepted Answer has become a part of that screen-section.