Neat stats! I think Toronto punches above its weight in immigrant diversity in part because Canada's largest city has a higher share and is a larger draw to immigrants proportionally than any one US city. Immigrants to the US are probably split among more cities than Canada, so we get a large concentration.Having looked at several North American metros, here's a listing of immigrant populations (of at least 5000) where Toronto is in the top 3.
Since these stats are for immigrants (I'm assuming means foreign-born and naturalized), I think we have for many ethnic groups, proportionally a lot more born overseas and having living ties to the old country. For example, say, in the case of Italian- and Chinese-Canadians, Toronto has a lot of people actually born in Italy or China, while New York and San Francisco probably has a lot of people of these descents respectively, but those that are second, third or fourth generation etc.