The 2009 Census estimate diverged dramatically from actual 2010 Census data, apparently overestimating Atlanta's population by over 20 percent. This article explains some of the causes of this gap. For example, Census estimates are based partially on the number of housing units, ignoring the impact of recession-related vacancies and shrinking household sizes.
From the Atlanta Journal-Constitution:
"Some responsibility rests with the city and county themselves, because they worked to drive up the census estimates in 2005 and 2006. They succeeded because the Census Bureau failed to push back. In addition, the census estimates understated the impact of the Great Recession on Atlanta and Fulton County."
Thanks to Michael Lewyn