If the average mark of a class for a test or assignment is much lower than normal, that should imply that it was harder than normal, and so grade boundaries should be adjusted for that. A teacher that refuses to shift their arbitrary grade boundaries, even if their inadequate teaching on a topic, sub-par test/assignment design, insufficient time allotted, and opaque marking methods might have contributed to this lower than normal grade is making their own job easier at the expense of their students. Of course the reason the marks were lower might be because most of the students had had a massive party just before a test, and turned up hungover, but otherwise being open to grade boundary adjustments can be a good thing for teachers.