I think this is the math concept bclintonk is getting to:
Suppose there are 10 classes at College A (and just to make the math easy assume each kid only takes 1 class).
Classes 1-9 have 10 kids apiece.
Class 10 has 90 kids.
10% of all CLASSES offered are 50+ (Class 10), but 50% of all STUDENTS are in a 50+ class (all the students in class 10).
It’s important to distinguish between number of classes and number of students.