Why does the accepted answer sometimes not show up first?

Question

On this question the accepted answer is not first. In this case the accepted answer has no up-votes, but I've seen this a couple times where there were up-votes too, so I don't think that's the cause. Why does this happen?

Answer 1

The accepted answer was posted by the person who asked the question. Not pinning the accepted answer under these circumstances is done to prevent gaming of the system.