Propublica has an interactive site which shows, based on Census figures, how equal or unequal the distribution of incomes in your county is. According to it, about 26 percent of populous counties are more equal than Fairfax,VA. Meanwhile, Prince William, the next further county out from Fairfax and home of Manassas, VA, more equal than every populous county. Meanwhile, places like Essex county, MA, Baltimore, MD, and Orleans parish, LA are up there in the 90+ percent.
I'm not clear why the differences, though my guess is: history. Fairfax has a longer history as a populous county than Prince William, so it had longer to develop pockets of poverty and pockets of wealth (McLean and Great Falls). The same of course is even more true for the old urban and suburban areas (Fairfax was mostly rural until the 50's.).