You're conflating school rankings with the quality of their graduates. While often school rankings indicate quality in that the ranking of a school corresponds to its selectiveness, it can hardly be said that
SMU is any less selective than
NUS, bearing in mind the fact that it recruits a smaller cohort of students each year.
In terms of the actual quality of its graduates, do you have any data to prove that
SMU grads are in any way inferior to
NUS grads? If you consider the 2017 JLC cohort, 60% of the local JLCs came from
SMU (despite
SMU having a smaller pool of students). Also, in terms of mooting achievements in "grand-slam" moots,
NUS's performance doesn't even come close.