This paper examines variability in teachers' usage patterns as they interacted with an online teacher support tool, the Curriculum Customization Service (CCS), in their professional work. Understanding variation of use with technology provides information about why users, and in this case teachers, use certain elements in their planning and teaching. By mining the usage log files of over 40 teachers who used the CCS over a year-long 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, using their log files with interview and observation data, we distilled "contextual contingencies" that influenced how they chose to use the CCS.