Bidens odorata Cav. is a synonym of B. pilosa in POWO:
http://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:185110-1

copying over comment from @bodofzt:

In Mexico, CONABIO chooses to maintain the Bidens pilosa complex as separate species:
B. alba: Up to 8 ligules that are notably longer than wide. Relatively short external (green) bracts. See http://www.conabio.gob.mx/malezasdemexico/asteraceae/bidens-alba/fichas/ficha.htm
B. pilosa: Absent or reduced ligules. At least some fruits with three awns instead of two. See http://www.conabio.gob.mx/malezasdemexico/asteraceae/bidens-pilosa/fichas/ficha.htm
B. odorata: 5 ligules that are barely longer than wide. Longer external bracts. See http://www.conabio.gob.mx/malezasdemexico/asteraceae/bidens-odorata/fichas/ficha.htm
See also: http://www1.inecol.edu.mx/publicaciones/resumeness/ABM/ABM.32.1995/acta32(85-89).pdf

POWO does not indicate what is the reason or based on what document is that it does not accept this species.
I need to investigate thoroughly what is the current status of this complex to give a better opinion, although personal mine is that they should all be one.