Using the same container volume with less net weight, for example. Those of us who have the luxury of not being particularly busy can certainly keep logs of unit pricing, but it's not practicable for most.
All the stores I go to show unit price on the labels. What difference does it make if something costs less last week? It is what it is now, so all a buyer can do is compare the options on the shelves, and choose one or not choose at all.