Background: As mentorship programs variably impact STEM identity formation, significant attention is paid to whether mentors and mentees are well-matched. Mentorship-matching studies focus on the salient ethnic and gender influences on the identity formation of underrepresented minorities. We study socioeconomic-matching influences to ascertain whether less salient attributes have a similar impact on mentors and mentees. Beyond demographics, we consider the (mis)matching of uncertainty avoidance values as deep identity influences on STEM identity formation. Our approach is novel, applying identity grafting (IG; blended, integrated, repressed, and reversed) theory developed by the first author to examine the mentorship (mis)matching underpinnings of mentors' and mentees' STEM identity formation. Results: We adopted a case study approach to interpret mixed methods data collected from an interschool STEM competition in Hong Kong, correlating mentors' and mentees' survey scores to sample a team for observation. Unlike salient attributes such as ethnicity and gender, socioeconomic status did not yield the positive benefits reported in the literature. Although sharing similar demographics, even unobvious ones, gave the mentor and mentees an initial rapport, this rapport was not sustained. The mentor and mentees were unaware of each other's socioeconomic backgrounds, and they struggled with mismatched values and expectations in their mentorship. The mentor wanted the mentees to solve real problems rationally, like engineers, while mentees wanted an inspiring vision. Mutually repressing themselves and "reversing" the new information received from one another, they grafted STEM identities that embedded the perspective that being rational and being visionary are irreconcilable. Hence, the team was unable to "integrate" their differences or blend shared attributes. Conclusion: Identity-matched mentorship is more likely to generate empowering experiences, and mismatched mentorship is more likely to generate innovative experiences. Although matched mentorship can improve rapport between mentors and mentees, mismatched mentorship, even in deep identity domains, can facilitate transformative learning for both mentors and mentees, depending on how team members reconcile differences. A paradigm shift is needed in thinking about identity and learning about differences through an IG lens. Schools can prepare children early for diversity by supporting this shift in how mentees and future mentors can feel comfortable experiencing a range of emotions, including excitement, distress, ambivalence, or a blend of emotions, during learning.