The Times Saturday had a report on the results of a new German census which cuts the German population. Germany had thought they had a handle on their population because of their mandatory registration system, but the first census in many years showed different.
According to the article what happened is that immigrants registered themselves in a place, which was added to the cut. But when immigrants decided to leave Germany, they often didn't report their leaving to the authorities, meaning the total population was inflated. What's more, because those shadow people were never reported as having died, it came to seem that immigrants were healthier than native Germans--the "healthy immigrant" paradox.
What's interesting is that scholars have worked on the "healthy immigrant effect" in this and other countries, offering varying reasons for the phenomenon. Google the term and see. So I wonder whether there's similar problems with the data being used to assess the effect in the U.S.?