Maybe because Manhattan is actually Kings County New York, and not technically part of the geographical New York City. While run as one city, (New York)it is my understanding that each of the five boroughs of New York are each one county, one legal entity, in the State of New York.
It is my experiences that FTM resolves places in the US in to subdivision, township, county, state. Since Manhattan is the subdivision and to my knowledge there are no townships, FTM resolves it to Manhattan Subdivision, Kings County, New Your State, USA
This is the same for a community in Ohio. where Chatfield Ohio is resolved into Chatfied subdivision, Chatfield Township, Crawford County. Ohio State, USA