This was my exact experience during my first two years in undergrad. It was especially frustrating because my grade was varying from an A to a C based on what felt like dice rolls. I normally ended up just thinking "oh I had bad luck on that one."

It took me until my junior year to actually figure out what it took to succeed in college after breezing through high school. Since the number of exams your grade is based on is small (normally 1 to 3), I feel there is still a luck component, but you can really tilt the odds in your favor through good prep work.