This paper highlights the importance of considering the context of effect measurements in development projects for education, particularly teachers’ access to ideas outside the project. Our case is a Swedish professional development project for mathematics teachers (Boost for Mathematics), which ran between 2013 and 2016. Its effects on teachers and students have been studied quantitatively several times. We examine teachers’ access to ideas through a qualitative analysis of a large selection of textbook series published between 1995 and 2016. Our results indicate the proposed ideational innovations of the project were not innovative. Teachers outside the project had good access to these “innovations” via textbook series. This result contradicts ideational factors as causes of measured effects, making organisational factors more plausible. The result also provides a new explanation for why measured effects were small. Our study exemplifies how qualitative methods can deepen the understanding of quantitative results and clarify causal relationships.