That is because http://www.irs.ustreas.gov/prod/forms_pubs/pubs/p5190606.htm
indicates that
Resident at end of year. You must file Form 1040, U.S.
Individual Income Tax Return, if you are a dual-status
taxpayer who becomes a resident during the year and who is a
U.S. resident on the last day of the tax year. Write "Dual-Status
Return" across the top of the return. Attach a statement to your
return to show the income for the part of the year you are a
