I find that my grades rarely measure what I expect.
How many times have you gotten a grade back where you did poorly even though you had studied for a week straight? Or what about when you ace that quiz that you completely forgot was even scheduled?
Probably only about 5% of my time in physical therapy school have I thought, wow, that exam/project/quiz/assignment grade accurately reflects my knowledge on the topic and the amount of work I put into it.
Recently, I failed a pretty important assignment in a class.