I'm surprised Wake/Johnston are not higher than Cabarrus and Brunswick. http://www.wral.com/news/local/story/2606977/
Wake & Johnston are much bigger, it's just the way the numbers work out. If a county has a population of say, 8,000, then every 80 new residents equals 1% growth. For a county with 100,000 population, it takes 1,000 new residents to make a 1% increase.