I think that by the time you’re hitting the higher level exams, a number of students have given up – sadly.
In addition, students at this higher level realise that it’s not always a good thing to over-face yourself and try 3 or 4 papers. Therefore they restrict their effort to just 1 or 2 and can focus their attention on a much smaller target (2 exams and not 3)
Thinking about that, it means that if they spend the same overall length of time studying 2 instead of 3, their effort on each of those 2 is 50% more compared with the effort for each of 3 exams