As said in the OP:
"Ten companies were formed of European immigrants in New York City and Washington D.C.; three companies consisted of Germans, three of Hungarians, one of Swiss, one of Italians, one of Frenchmen, and one of Spaniards and Portuguese."
However, looking through the roster, it looks like there were a fair amount of people in the companies not associated with their ethnicity. Many, but few in comparison, Americans ended up in the 39th New York as well, filling in the gaps, I guess.
It's just kind of funny that it's so Italian-centric with only roughly a hundred Italians in it.