It is at least partially due to the education system: MPSI -> MP -> ENS Cachan / Lyon / Ulm. Very hard entrance exams to make sure they get the best students. S

It is at least partially due to the education system: MPSI -> MP -> ENS Cachan / Lyon / Ulm. Very hard entrance exams to make sure they get the best students. Small promotion (IIRC around 40 students per year for ENS Ulm FIMFA) = homogeneous level.