Inside the World’s Most Surprising School System, via a kind reader’s email.
The PISA survey tells only a partial truth of Finnish children’s mathematical skills:

The results of the PISA survey (http://www.jyu.fi/ktl/pisa/) have brought about satisfaction and pride in Finland. Newspapers and media have advertised that Finnish compulsory school leavers are top experts in mathematics.
However, mathematics teachers in universities and polytechnics are worried, as in fact the mathematical knowledge of new students has declined dramatically. As an example of this one could take the extensive TIMSS 1999 survey, in which Finnish students were below the average in geometry and algebra. As another example, in order not to fail an unreasonably large amount of students in the matriculation exams, recently the board has been forced to lower the cut-off point alarmingly. Some years, 6 points out of 60 have been enough for passing.
This conflict can be explained by pointing out that the PISA survey measured only everyday mathematical knowledge, something which could be – and in the English version of the survey report explicitly is – called “mathematical literacy”; the kind of mathematics which is needed in high-school or vocational studies was not part of the survey. No doubt, everyday mathematical skills are valuable, but by no means enough.