Talk to these guys: http://www.investopedia.com/articles/personal-finance/050615...
Scrolling through all their infographics, I finally get to this: "For the income track, we've used the most recent (2008) statistics from the World Bank, based on household surveys. Here we rank you against the entire world population at the time of the surveys, estimated at 6.69 billion people."
I dug into the data behind your claim, briefly, and I think I ended at a self-reported survey. So, that data is probably wrong.