With any operation as large as the United States census, there are bound to be some mistakes. Some people are counted twice. Others are not counted at all. The difference between the actual population and the number who are actually counted is referred to as the net undercount. The undercount has a disproportionate effect on certain groups.
Children are much more likely to be undercounted than adults.
African-Americans and Hispanics are much more likely to be undercounted than Anglos. Renters are much more likely to be undercounted than homeowners. The redistricting or PL 94-171 numbers contain the basic information used in redistricting but do not include the more detailed data gathered by the census. Basically, the redistricting information will show at the census block level the total number of inhabitants and the number of voting age inhabitants. It will also break down this information by race and by Hispanic status.