A Census Consensus

As of July 11, World Population Day, there are 7.2 billion people on this planet. It is projected by that the year 2050, we'll be at ... well, the U.N. and the Population Reference Bureau say 9.6 and the U.S. Census Bureau says 9.4. How do they know this, why can't they agree, and why, in 2000, did the U.N. predict that the number would be 8.9 billion?: http://news.nationalgeographic.com/news/2013/13/1307011-population-census-united-nations-un-demographics-world-population-day-birthrate/