Educational psychology studies have linked self-belief constructs to success in STEM, including self-efficacy (the belief in one's ability to succeed at specific tasks). However, most of the work with these constructs uses traditional psychology large-N quantitative studies to show that change happens without describing in qualitative detail the mechanism by which it happens. This paper will focus on a case study of a student who sees herself as competent in neuroscience, identifies as a math person, but still places physics in a category of special difficulty. Evidence from this case suggests that receiving grade-based evidence of her capability in physics is insufficient for impacting her self-beliefs in physics. We explore ways in which a nuanced understanding of how a sense of capability develops helps us understand this student's disciplinary self-efficacy.