This is most likely due to caching. The OP accepted an answer, then changed his mind and un-accepted. In fact, in my browser, the question is not shown as having an accepted answer:
When an OP accepts, then unaccepts an answer, the 'accepted' status of the question can still be cached in listings. You'd have gained, then lost 15 points, which if that occurred within the same day, is not listed in your reputation.