A lot of professors in my field (physics) see tests as a learning tool where one is presented with new things to be figured out. In my undergrad, exams were typ

A lot of professors in my field (physics) see tests as a learning tool where one is presented with new things to be figured out. In my undergrad, exams were typically open-book and consisted of a few long, multipart problems. It does not make sense to scale such exams to 100%. Anyway, by setting the expected mean to around 50%, you get a more accurate evaluation of people's understanding (by minimizing truncation at the limits).