American calories consumed and exercise during growth has something to do with it. I'm surprised how little study is done on how certain foods and exercises affect different kinds of growth. This is an important subject because once the growth stops the concrete is set for life.
Definitely good American nutrition is a factor for these immigrant children. Look how many lower class poor Vietnamese children who became HS valedictorian in America.
In Vietnam the highest they could have gotten was rice farmer. Here he can get hired by Lehman or AIG and take home millions trading credit default swaps