Isn't some of it have to do with it being a self selecting sample? If you come to America to study, you were a good student in your resident country leading to more chances of success than the average local citizen. Their children might be smarter on average. Alternatively if you are coming fleeing persecutions you are enterprising enough to set up something for your children. That hard work inculcates a sort of work ethic in such children which in turn sets them up for success. Speaking from experience as an immigrant myself.