If there is a 3PM class and an 8AM class, 90%+ of your people are going to sign up for the 3PM class (commuting students might not if they can use the early classes to pile all their classes into the same day).

Normally, if you needed a class to graduate, you could generally work something out. However, those classes were generally "in your major" and mostly never banged into hard limits (laboratory classes being the exception).

I am reminded of my beginning foreign language classes. They were always oversubscribed, but if you showed up the teacher would always sign an exception for you. They knew that at least 25%+ of the class was going to drop, so, they could basically give everyone an exception who showed up to the class.