It's just that the majors are different. Some have many courses, some have few courses, some have many courses when they are freshmen and sophomores, and some have many courses when they are sophomores and juniors, but they have never attended classes. For people with few courses, it may be only three or four days a week, and then leave a lot of spare time for their own control, such as going to the library, participating in club activities, doing part-time jobs and so on.