Curriculum ideals often get compromised as a curriculum moves through its various levels of representation. Across the different science reforms, this process of slippage is clearly present. Research shows that teacher subject matter knowledge, PCK, beliefs and context factors all influence implementation. Professional development arrangements focus on fixing deficiencies in teachers' knowledge, beliefs or work context. This approach has not solved the problem of slippage, as we still do not understand what mechanisms operate when teachers make decisions about change proposals. In this study, we unpacked the decision mechanisms of three highly qualified chemistry teachers who subsequently adapted an innovative context-based chemistry unit. In spite of a state of the art professional development arrangement and the teachers being highly qualified, slippage occurred. The teachers' goal systems were used to interpret their reasoning. A goal system is a context-dependent, within-person mental construct that consists of a hierarchy of a person's goals and means in pursuit of a task. We showed that all three teachers adopted or redesigned elements of the change proposals to meet their core goals, i.e., goals that had multiple connections with other goals. This indicated that the adaptations teachers made were perfectly reasonable ways to serve their professional goals. For change to happen, we contend that one should begin with ways to connect teachers' core goals with the lesson or unit goal demands of a proposed innovation. Change emerges from the adaptions teachers make in the service of their core goals.