Some of this is in the eye of the beholder. I've seen people working 10+ hours in a class and still end up with an A, but found the process mentally exhausting. Others might barely work at it and end up with an A-, and think that it was pretty easy.

At least in my experience as a math major undergraduate, calculus was a class that introduces students to rigorous proofs and new lines of thinking, but in examination seldom tests students on proof-ability. Later classes such as Real/Complex Analysis are where you are expected to produce proofs during examinations. So I would not be surprised if some people 'saw the writing on the wall' in that they could solve problems on examinations but were lost on the theoretical content.

Oddly enough, (also anecdotal) I'll mention that STEM majors in the hard sciences and mathematics seemed far less obsessed about GPA than many other majors. So a Bio major (especially if they were pre-med), for instance, would be much more stressed about upcoming examinations than the math or physics folks.