Luck vs. Preparation in CSS

[Otaku]

Many experienced candidates have argued that qualifying CSS is a matter of luck, but theoretically speaking, CSS is an exam, not some lucky draw. There are questions that the candidates have to answer. If a candidate can answer those questions correctly, i.e. provide all the required information in a well-organized manner, then the examiner should mark the candidate accordingly, give him a good score, and the candidate should pass the exam with flying colors. However, there seems to be a difference between theory and practice, because many candidates complain that they wrote perfectly correct answer and expected to pass, but still failed. This leads to the question that can a candidate pass CSS through sheer preparation, or is luck a prerequisite for qualifying the CSS exam.
Please share your opinion on this matter. Although you are free to express your opinion, please do not argue that both preparation and luck are required to pass CSS, because that would imply the inevitability of luck which renders preparation meaningless.