He says
Each year there's several papers around the world and because of that they look at all the scores and how hard it is for each paper and run a statistical test providing a score for critical thinking, problem solving and an overall score.
That score is not necessarily out of anything but the higher the better. It used to be that if you got say 38 in the 2008 papers you wouldn't get a place as the statistical score would have been lower than the ones for more recent years, the scores aren't meant to be comparable scores on different years but to the scores for the different papers round the world,