Washington grows by 81K people between 2008 and 2009

[jimrtex]

State estimates generally exceed the Census Bureau's numbers. Unless there is a substantially different method for the Census to do its enumeration, I would tend to favor an estimate towards the Bureau's value. However, the point is accurate that any state near the threshold for a congressional seat has a great incentive to get everyone counted that they might.

Some places in the US have an even bigger discrepancy. For example the city of Surprise in AZ:

The city had 7.000 inhabitants in the 1990 Census and 31.000 during the 2000 Census.

The Mid-2008 Census Bureau Estimates are at 92.000, but the City demographers as well as the AZ state demographers put the number at 109.000 and about 112.000 for Census 2010 ...

Though I recall a newspaper article that said that the state estimates for Surprise may have been too high.  Surprise is the westernmost major suburb of Phoenix.  State estimates typically use dwelling unit counts along with estimates of occupancy, and there had been some overbuilding in Surprise so that you had vacant houses and apartments.

The census bureau has fairly reliable data for births and deaths, though there may be some misattribution of location.  But their migration data relies a lot on income tax and social security data, and so may miss people who don't pay taxes.

[jimrtex]

General question:

Do any US cities and towns require people to register at the town hall when they move from one city to another ? Or do cities generally not have population registers ? Not to mention a general US-wide population register ...

I've never heard of something like that.  If there was, it would be a useful source of genealogical information.  Perhaps there was something like this in 1600s in Massachusetts, where there was more of an interlocking of the Church and State.

In most of the United States, local administration of state law is through the counties, and counties will have completely covered the state by an early time.  Property transfers would be recorded at the county level, but owning a property and living on it aren't quite the same.

Probably the closest thing the US has is a driver's license.  That is why the law requiring States to solicit voter registration at any government office is dubbed "motor voter".  Essentially everyone would have to get a driver's license and thus could also be registered to vote at the same time.