There are many reasons people get overlooked when the census is taken, every ten years.
Some believe anything the Federal government does is against their interests. Some believe that anything the Federal government does is illegal. Some, following 2 Samuel 24:2-15, believe participating in the census is a sin; there’s no law requiring people to be counted, only a clause in the Constitution requiring the government to count.
However, the Americans most likely to be counted are native-born wealthy white citizens. That means native-born wealthy white citizens have an influence on policy out of proportion to their already substantial numbers.
Some people object to this (few of them native-born wealthy white citizens, obviously). They would like to see sampling techniques incorporated into the census, which would counterintuitively result in far more accurate numbers than a simple head count. A 2002 Supreme Court case, Utah v. Evans, ruled unconstitutional the use of census figures arrived at by sampling techniques in determining Congressional districts, but there are many other things census data is used for.
That’s why the Obama White House intends to work closely with the Commerce Department (while not taking direct control of the Census Bureau). With White House supervision, they can count everyone accurately (as well as tabulating all married couples, not just most) inteads of skipping some people.
If no one knew there was an undercount, it would be unavoidable. Since we do know, however it’s unconscionable to allow it to go unremedied.