Other individuals do not file because they do not have an income to report. Finally, some people simply do not file for personal reasons. Many people maintain a permanent residence in one county or state but live in another. These people pay taxes in a different county or state than where they live. Again the Census and IRS will account for these people in different locations.
Many people file their tax returns late.
Thus, the data from the 1991 IRS files may not include certain families that would be included in the Census count.