I've read time and time again that, at least in Japan, test scores are so high because the federal education system focuses almost exclusively on preparing students to take said test.
Supposedly its because when Asians that are used to strict standards like move to the US, they retain those standards. Then they have a kid, and the child is taught to live by the same standards, and it goes on for a few generations. At least, that seems to be the popular answer. Its always worth noting that this is obviously why the "smart asian" generalization exists in the first place.