Contemporary coaching and leadership discourse frequently treats achievement as evidence of personal growth, psychological maturity, and developmental advancement. High performers are often assumed to be more self-aware, emotionally integrated, and developmentally evolved than their lower-performing counterparts. Yet this assumption remains largely unexamined within coaching theory and practice. This paper argues that such thinking reflects a conceptual error, referred to here as the Performance Fallacy: the conflation of observable performance with underlying developmental maturity. Drawing upon developmental psychology, constructive-developmental theory, and coaching philosophy, the paper distinguishes between performance and development as fundamentally different constructs. Performance concerns the production of outcomes and the demonstration of effectiveness, whereas development concerns the structure through which individuals make meaning of themselves, others, and the world. While development may contribute to performance, performance alone cannot reveal the developmental architecture from which achievement emerges. The paper argues that identical levels of achievement may arise from profoundly different psychological structures, including both integrated and adaptive forms of functioning. As a result, coaches who equate achievement with development risk reinforcing existing adaptive patterns rather than facilitating genuine developmental growth. The paper concludes by proposing a developmental reorientation of coaching practice, one that shifts attention from the optimisation of performance toward the examination of the structures that organise it.