In this article, we describe the case of "Keri," a fifth-grade teacher who had completed an Elementary Mathematics Specialist (EMS) certification program. Drawn from a larger study investigating the knowledge, beliefs, and practices of EMSs, Keri's case was unique in that she was teaching mathematics to four classes in a departmentalized structure, where students were placed into different classes according to perceived mathematics ability. Observations from the larger study revealed that Keri's instructional practices did not align with her reported beliefs and knowledge. To explore this deviation, we conducted a case study where we observed Keri's instruction across multiple classes and used interviews to explore reasons for Keri's instructional decisions in terms of her perceived professional obligations. We found that Keri did employ practices that were aligned with her reported beliefs and knowledge such as pressing students for mathematical justifications, but only in her "higher ability" classes. Interview data suggested that Keri's decisions were driven by a strong obligation to individual students, overriding other obligations. We describe implications of these findings, including the limitations of teacher assessments and surveys as proxies for teaching quality, and discuss recommendations for approaches to teacher development that account for teachers' perceived obligations.