I'd say the self-selection bias would be a big factor. Most of the dumbass and lazy Asians probably stayed at home. Those first generation asian-american kids would likely have had the value of hard work forcefully instilled in them as well.

I have taught in Japan. Their dumbasses say, "hi."

(Seriously, "the Asians = smart" myth doesn't last long if you actually meet normal people in Asia. Normal Asian people are... of normal intelligence! What a shock!)