Northwestern’s 171 Visual/Performing Arts grads is far more Visual/Performing Arts than at any of the other 24 colleges composing the sum, even though Visual/Performing Arts is a less popular major than Economics and various others. The comment is meant to show where the Visual/Performing Arts grads are coming from, as it is not a consistent distribution across all 25 schools.
You can view the full breakdown for a different year at College Navigator - Northwestern University
Drama = 68
Music = 21
Stringed Instruements
…
Total Visual & Performing Arts = 169