Although the facts presented are true, you have to take a look at what's behind the facts before making an accurate observation.
Asian culture is different than that of the United State's culture. One example is that in Hong Kong (China in general) there are a lot more people in that country than in the United States, therefore the competition for jobs is much greater. For most Chinese citizens, the only way to get out of poverty is to find a good job, therefore they focus more on their studies to out-compete others.
Also, many Asian countries offer tutoring outside of school. So even though Asians spend less time in school, they spend additional time in tutoring to enhance their knowledge and testing skills.