I could be wrong, but I am almost positive it has nothing to do with seat numbering. The way it is supposed to work, is that First/Business will board first. Then Gold, Platinum and Diamond medallions will board (Zone 1) regardless of their seat assignments. Silver Medallions and those with the AMEX credit cards board with zone 2. And then I think zone 3 is limited to a small group of people that doesn't have anything to do with seat numbering