When would they say a person has an eating disorder nos?

It's a technicality. The diagnosis ednos just means that the patient doesn't fit neatly into a dsm-iv diagnostic category, it has little real meaning as the symptoms are the same or similar and the mortality (death) and morbidity (illness) is not really different.
See below. Nos (not otherwise specified) means there are symptoms of disordered eating, but that there is not enough information at the time of diagnosis to better specify the type of disorder, or that the disorder doesn't quite fit the diagnostic criteria for something specific. This diagnosis still suggests, however, that eating is somehow disordered and treatment indicated.