There's another principle at play. Colleges aim to maximize their yield, or the percentage of students who are offered admission that ultimately matriculate. Nothing against CMU - it's a fantastic school in nearly every regard - but these perfect scoring students... are they most likely to attend CMU? In a lot of cases the school is competing with HYP and others of that ilk.
So if you're CMU admissions, how do you maximize yield in those scenarios when P(student matriculates) is relatively low? You reject the student in favor of the better (if lower scoring) fit. Not only does it enable them to better predict/manage the size of the incoming class, but it also increases yield, which is a number that's reported directly to US News and which (in principle if not in practice) describes the desirability of admission from the perspective of incoming freshmen.
So if you're CMU admissions, how do you maximize yield in those scenarios when P(student matriculates) is relatively low? You reject the student in favor of the better (if lower scoring) fit. Not only does it enable them to better predict/manage the size of the incoming class, but it also increases yield, which is a number that's reported directly to US News and which (in principle if not in practice) describes the desirability of admission from the perspective of incoming freshmen.