Questions without answers don't count against your acceptance rate.
It would be unfair to penalize someone for not accepting something that doesn't exist.
This my also apply if all the answers are negative, but I'm speculating.
First, the accept rate is heavily cached. Second, it excludes questions that are new (less then two days old), have no answers, or are community wiki.
7 questions have answers.
4 are marked as accepted.
4/7 = 57.14%