There are two reasons why the “mean of department” or other comparative means might change. Usually, the differences are within a couple hundredths of a point, and not statistically significant:
Corrections, made at the request of the department chair or dean, to other courses in the department. Making these corrections necessitates a recalculation of the department-wide means for all courses in the department. Differences between the new "Blue" survey platform and the old survey processes on paper and in Sakai that cause some questions in team-taught courses to be weighted differently in each system (per-section versus per-instructor). Importing the data from Blue into our local historical database causes the Blue data to be recalculated using the older weightings (multiplying the responses by the number of instructors in the course). This only affects questions about the course or student, not the questions about the instructor.