This paper examines variability in teachers’ usage patterns as they interacted with an online teacher support tool, the Curriculum Customization Service (CCS), as part of their professional work. The CCS is a web application that supports teachers in planning, adapting, sequencing, and enacting differentiated instruction in Earth science education. By mining the usage log files of over 40 teachers who used the CCS over a yearlong period, we analyzed for variability using a framework developed in marketing research to characterize appropriation of technology. This analysis helped reveal different kinds of teachers’ patterns along two dimensions: frequency and variability of use. We then turned to qualitative records of teachers’ experiences during the year to better understand why those variations appeared. Focusing on the experiences of several teachers, we distilled “contextual contingencies” that influenced how they chose to appropriate and use the CCS.