There is a contradiction of intensive national policy efforts and the slow pace of change in widening participation in England. This paper focuses on the use of contextual data in STEM subjects, where there has been less progress in widening access and a more rigid entry pathway through A-level study. Interviews with admissions tutors suggest a narrative in which STEM curricula, and the prior knowledge and skills required to succeed within it are fixed, and that attempts to widen participation and broaden the notion of 'best students' could undermine academic standards and the student experience. The paper draws on social justice and social reproduction theoretical frameworks to explore policy enactment, identifying support for widening participation and the effects of a conservative ethos amongst academics. The tension of these approaches, alongside the autonomy of decision-makers, are key dynamics explaining the inertia of policy efforts for change.