MacBooks tend to last a lot longer than Windows machines. I'm not talking about build quality here (although that also helps) but about useful lifetime. Any MacBook or MacBook pro built in the last four years can still run the most recent OS, and is therefore good enough to run any software your university course may want you to use. This definitely isn't true for Windows laptops, and especially not for low-end Windows laptops: you'll have a hard time running Win7 on a two year old budget Dell or Acer.
So a lot of these students could be using the hand-me-downs of their older siblings, parents, whatever.