If your ancestors moved or immigrated with their children, is it possible that an older child stayed behind without making the journey with his or her parents? The oldest child in a family may have been married or gainfully employed when his or her parents decided to move. Sometimes these children would eventually settle where their parents did, but often they did not.
One relative and his three youngest children moved from upstate New York to Chicago in the very early 1900s. His two oldest daughters remained in New York. Don't assume the entire family moved together.