Did they changed the answers sort order recently ? It seems that the "best" answer is not on the top anymore... (Or maybe is a bug ;-)
If the accepted answer is also by the same user who asked the question, it will not be stuck at the top of the list of answers, regardless of sort option.
It will instead appear wherever it would in the ranking by votes or time given the tab you've selected.
You have the option to sort the answers however you like. It's in a series of tabs, right underneath the question.