As mentioned by Jeff, it uses the 0-upvoted answers definition. The community user will not bump a question if there are any answers with a score > 0. As for why this is preferred, let's look at it from both sides:
At least one answer, total score is 0 or less
Let's pretend for a moment that the question does not have any good answers. In that case, isn't this question just as deserving of extra attention as one with no answers at all? On the other hand, say it does have a good answer. In that case, it only takes one upvote to that answer for it to disappear from the unanswered view and never get bumped again. We want to make sure the unanswered view is well maintained, and so the chance to upvote this answer is just as valuable to the community as bumping a question with on good answers.
Your argument here is that these questions are somehow more deserving of the extra attention. I put it to you that there is often a very good reason these questions are not answered. Often, the only purpose of bumping these questions is so we can spot them and vote to close them. In other words, they are just as often less deserving of your time, not more. I will grant you that a filter on the score for the question might fix this, so that it only bumps questions with a score >= 0. Then it would also provide the community a chance to downvote or close these bad questions in the same way it gives us a chance to upvote deservings answers to clean the unanswered view.