北维州两个counties不是连续多年是全美收入最高的吗？怎么回事啊？

10 highest-income counties by median household income (2007) [1]
Number of counties by state in following lists: Virginia 6, Maryland 5, New Jersey 3, New York 2, California 2, Colorado 1, Georgia 1.
10 of the 20 counties are part of the Washington Metropolitan Area and 5 counties are part of the New York Metropolitan Area. By Census region, 12 counties are located in the South, 5 in the Northeast, and 3 in the West. 16 out of the 20 counties are part of the densely populated Northeast Megalopolis.
Counties with populations of 250,000 or more
Rank County Median Household Income
1 Loudoun County, Virginia $107,207
2 Fairfax County, Virginia $105,241
3 Howard County, Maryland $101,672
4 Somerset County, New Jersey $97,658
5 Morris County, New Jersey $94,684
6 Douglas County, Colorado $92,824
7 Montgomery County, Maryland $91,835
8 Nassau County, New York $89,782
9 Prince William County, Virginia $87,243
10 Santa Clara County, California $84,360