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You seem not to have not only read the article, but not read the title of the article. The entire point is that not only are US children of parents with low parental education behind countries like Portugal, Estonia etc., but US children of parents with high parental education are behind Portugal, Estonia etc.

The analysis of this study dovetails with other analysis of PISA rankings, about how the top 10% of students in Japan and Korea have significantly better math proficiency than the top 10% of math students in the US, how their top 20% is better than the US's top 20%.

This is what the data says and the analysis of the data. The typical reaction is of your type - you don't even read the article title, assume it is saying average rankings are below, and repeat the Panglossian idea that the US children of the well-educated who are ahead of most US students are on top of the world, or near it. The data shows otherwise. Even non-stringent observation shows otherwise, if you take a look around at who is getting doctorates in mathematics and the like at top American universities.