Having gone through the Canadian education system, I would guess that Canada's high score comes from doing a good job on the lower percentiles, 1-50 or so. The system is slow and repetitive in the lower grades; its focus is getting everyone up to basic standards rather than challenging the best students. But if you're at all clever and diligent, it's boring.
Tracking happens late, at entry to high school (grade 9), where they split you up into academic, regular, and vocational tracks. From there on things move at a more respectable pace, at least in the academic track. But at that point the top-track students are already years behind their peers in other systems that moved faster and tracked sooner. I would guess the top students coming out of high school are about two years behind students in academic tracks in other countries, like the UK or Germany.
Tracking happens late, at entry to high school (grade 9), where they split you up into academic, regular, and vocational tracks. From there on things move at a more respectable pace, at least in the academic track. But at that point the top-track students are already years behind their peers in other systems that moved faster and tracked sooner. I would guess the top students coming out of high school are about two years behind students in academic tracks in other countries, like the UK or Germany.