If there’s one thing I’ve learned with 100% certainty between high school, college, medical, and graduate school, it’s that you can never trust someone’s self reporting of their studying. Not everyone will skew in the same way, but almost no one did whatever you think “3 days of studying” means. Some people did “3 days of studying” but they’ve been reviewing class notes and the textbook every day before and after class the entire semester. Some people did “3 days of studying” but that means they opened the textbook for the first time 3 days before the exam and spent 2 hours each day in the library and 85% of the time they were on Facebook.
As @UCBalumnus points out, the “0-100” scale that most high school students are used to is a pretty bad scale. Failing is often under 60 so you’re really only left with 40 points of room to differentiate people who didn’t fail (and only 10 points for each letter) in contrast to 60 points of room to differentiate the students who did fail - you don’t need that much stratification of bad students. In many schools, many profs want to differentiate great students from truly outstanding students so they make the grades overall lower because it gives more room at the top where an outstanding student can pull away from the pack and similarly, gives them more room to work with in separating the passing students in general.