This study undertakes a dual exploration, examining the effects of individual differences on self-assessment accuracy, and students' perspectives on the reasons for their inaccuracies. A total of 112 psychology students self-scored an essay, then received the instructor's score and were interviewed. Results revealed: (1) a weak positive correlation between self- and instructor scores, with most students slightly underestimating; (2) a significant effect of essay score on accuracy, as higher achievers tended to underestimate and lower achievers to overestimate; (3) a small but significant effect of learning-oriented self-regulation on accuracy, with higher scorers tending to underestimate; (4) inaccuracies were rarely attributed to assessment criteria but instead reflected internal perceptions; (5) high achievers justified underestimation through pessimistic self-perceptions, whereas low achievers explained overestimation through optimism and overconfidence. The findings reveal that self-assessment is as much an affective process as it is a cognitive skill.