There are claims that sampling errors for the best known one hurt the US. (See http://en.wikipedia.org/wiki/Programme_for_International_Stu... for more.) I have heard anecdotes that suggest some Asian countries (notably Singapore) pick unrealistically good students for their sample to make themselves look good. And also there is a question about mismatch between the test and the curriculum - certainly the US does better on http://en.wikipedia.org/wiki/Trends_in_International_Mathema... than on PISA.

None of this is to say that the US actually has a good education system. But it is not necessarily as bad as people think.