As the U.S. Census Bureau sees it, Hanson County, South Dakota, grew like it was on steroids between 2003 and 2004. The bureau claims that rural Hanson is the fourth fastest-growing county in the nation. But local residents don't get it. They haven't seen indications of the supposed 7.9 percent increase in population. And you'd think they'd notice such a thing in a place with only about 3,800 people to begin with. Clearly, the new "residents" aren't sticking around for the welcome wagon or homemade apple pies from the neighbors.

It turns out there's a business in the town of Emery, in Hanson County, called My Home Address Inc., that provides a service to people who spend most of their time traveling in recreational vehicles. RV nomads need to register their existence somewhere, and South Dakota has only a 3 percent motor vehicle excise tax to get license plates and the tabs that go on the plates. Plus, the state has no income tax. So people who live in their RVs full time come from all over the country to sign up with the company. That gives them a place to receive mail and create a "home" address. It is also the residence they list on their tax forms.

It's not certain that My Home Address is the cause of the inflated population numbers--nobody from the bureau has officially investigated--but it's the best guess the townsfolk have for their phantom growth. Especially considering Emery's population jumped 21 percent between 2000 and 2004. "That's fairly high," says Nancy Nelson, the federal-state liaison for South Dakota.

Katherine Condon, demographic statistician at Census, confirms that the bureau does use IRS data to look at "internal migration" within an area. So if people with RVs are using a town in Hanson County as home, and paying taxes from there, a population spurt that comes without actual bodies makes sense.

Such oddities are not considered unusual to the pros at the Census Bureau. There are various reasons why population estimates get skewed, and it may take a couple of years to figure out why. For instance, a large nursing home may have been built or a prison may have added a new wing since the last Census and inflated an area's population count. In a rural area, that can show up as a shockingly large percentage increase in population.

Sometimes, the estimates are in error. A few years ago, Carlisle, Indiana, learned from the Census Bureau that it had a 300 percent explosion in population in a place that normally has only about 600 residents. No one in Carlisle had noticed that the streets were busier or that the lines were longer at the coffee shop. It turns out that that year there was an error in assigning the state prison population based on a ZIP code issue.

Legitimately large population increases often come from annexations of unincorporated areas, causing an entity to redefine its geographic boundaries, a move that gets the area additional funding for CDBG, welfare and transportation based on the population formulas. But when growth changes seem peculiar, it is the job of a federal-state liaison to figure out what's going on so the estimates can be reviewed before they are released. If a jurisdiction believes the estimates are incorrect, there is a formal process for challenging the numbers.

Sometimes temporary events skew estimates for a couple of years. In the case of hurricanes, floods and other natural disasters, entire neighborhoods may be wiped off the map, but tax records for that year may have been filed earlier from those flattened homes. "People are displaced and suddenly appear where FEMA [the Federal Emergency Management Agency] has set up a lot of trailers," says Greg Williams, Alaska's state demographer.

By now, at the mid-point of the decade, most mix-ups from the 2000 Census have been straightened out. "A lot of red flags have already been taken care of," says Vincent Thompson, economic research analyst and federal-state Census liaison in Indiana. But then, in five years, it begins all over again. "Of course, there will be a whole slew of them after 2010," Thompson says. "There is some art as well as science to this problem."