Another way of looking at that same data is to look at all the students who scored around a 60% on the summatives assessments and look at the range of performace on the formative work. You could also pick all the sutdents who scored around 80% on the formative and look at the range of performace on the summatives.
Really this discussion could be in the product part of this case, but given that often HW is scored as process (did you attempt it or not), discussing it here in process is also appropriate. If you score on attempt it's really a process score. If you score on correctness, it's more of a product grade. Either way, HW is usually an opportunity for practice and more diagnostic in nature, so we really need to rehtink how it is used in calculating a grade.