How do you become a "Top Contributor"?

At first I thought it had to be your best answer to answer ratio, but then I saw a Top Contributor with a 13% best answer. I have a 17%. Then I thought it was the number of questions you answer, because this contributor answered about twelve hundred. But, when I compared this to a regular Y!A member, the regular one had a 64% best answer and thirteen hundred answered questions! How insane is that? Why isn't HE a Top Contributor?