It is vitally important to study and prepare as well as possible for an exam.
But studying to achieve understanding and mastery of course content is not a guarantee of being able to do justice to hard work and preparation when it comes to delivering in an exam room.
So studying may be thought of as a necessary but not sufficient factor in exam performance.
A host of factors that influence exam performance comes into play as the exam nears, as you enter the exam room, and as you turn over the exam paper to see the challenge facing you under exam conditions:  perhaps dealing with your expectations and pressures attaching; perhaps dealing with anxiety levels that are too high; perhaps dealing with concentration issues arising from anxiety, tiredness, or other factors; perhaps dealing with personal problems going on in your life outside of school and the exam process; perhaps dealing with time constraints; perhaps dealing with a poor understanding of exam management and performance under pressure.
Sports psychology has come to be an accepted and much sought after input for athletes - both amateur and, significantly, professional -struggling less with their ability to 'do' their sport but with their ability to perform when it matters:  in a real race; when the penalty is to win a match or a cup final; when a double fault will lose a match point; when the three-foot putt is for a championship.
The psychology of exams is every bit as helpful as sports psychology when it comes to 'performing' under exam conditions.