The average amount of food I buy should be, on average, the same as the amount of food I consume, but the amount of food purchased during a given shopping trip varies, especially in the amount spent on nonperishables. I am likely to buy a lot of a given item, store the extras for when I need them, and buy more when I run out.

If I am wandering around the store, I may see an item (like a snack) that I am interested in trying and pick some up. In doing so, I am slightly delaying the next time I need to buy more, but it is an overall gain for the store since they are getting my money earlier and the future stock up trip may be from a different store.