Family historians need to remember that for many censuses, we do not really know who answered the census questions. Was it the wife who never knew her husband's parents and yet had to answer questions about where they were born? Was it a child who had no idea when her father immigrated to the United States or when he became a citizen?
Most of us weren't there when the censustaker came to our ancestor's door. As a result, we just don't know who really gave the answers to specific questions. If the answers vary from census year to census year, it may be because the individual answering the questions varied from census year to census year.