numeric fields imported as factors by readOGR()

I often get some numeric fields (i.e, AREA in this case) imported
as factors by readOGR() from shp files. WHy could it be?
I've reviewed the field in the dbf file and do not
see any non-numeric value.

Thanks
Agus

Dr. Agustin Lobo
Institut de Ciencies de la Terra "Jaume Almera" (CSIC)
LLuis Sole Sabaris s/n
08028 Barcelona
Spain
Tel. 34 934095410
Fax. 34 934110012
email: Agustin.Lobo at ija.csic.es
http://www.ija.csic.es/gt/obster