I was listening to the latest episode of the One Piece Podcast, and Stephen (the official English translator) said that he got the chance to see the numerical tallies for the mid-term results. He said that while Zoro came in first in most territories, what surprised him was that in Asia the number of votes for Luffy was insane, way beyond any other character. So it seems Asia is singlehandedly responsible for putting Luffy at the top worldwide over Zoro.