The Asians present in North America are not a random sample. A large portion of them came here specifically to work in STEM or other fields requiring high levels of intelligence and education. It's very probable that the ones here are, on average, significantly more intelligent than both the average American and the average Asian in Asia.
For that matter, the Asians you see in studies of the average test scores in somewhere like “China” aren’t representative of the educational ability of the average rice paddy farmer, but of more urbanized populations.