You're assuming two things

1. That Notre Dame is 100% full at all times, that there is not room for any more students in classes.

2. That Notre Dame can predict to the last student how many of the people Admissions accepts will enroll rather than working with a range of the number of accepted students who will enroll. Therefore there is not an empty bed or room on campus.

But there is room for a few more students in almost every class. The marginal cost of adding the 26th student to a 25 person class is the cost of the textbook.

The percentage of accepted students who choose ND over other schools that have accepted them varies every year... varies within a tight range, but varies nonetheless. 2,000 might accept one year and 2,400 might accept the next year (example numbers, not data from Admissions). The admissions model leaves some room for an unusually high acceptance rate, so there are empty beds almost every year. Unless every bed on campus is full, the marginal cost of room and board is the cost of filling an otherwise empty bed, not zero but not the fully loaded cost of room and board.

I remember a couple of years in the 1970s that were so full that upperclassmen were forced off campus, but a lot of dorm capacity has been built since then.