Could the secret to Finland's excellent schools be...Finland's excellent students?
But perhaps it's this relative lack of individual ambition that allows the system as a whole to perform well? I.e. the Finnish school system delivers a high mean with fewer of the excellent outliers. (Personally I think that's a reasonable goal for an educational system because measuring students doesn't work all that well, so it would be dangerous to optimize for exceptional performance on a broken metric.)
The other part consists of students who are normally thought to be worse in school. In contrary to their lukio peers, they do not have matriculation examination and therefore do not contribute to the statistics shown on the article.