Perhaps we can shed some light on the situation here.
Grocery stores rely on just-in-time systems to get their food to their customers. On average, this means that a grocery store has roughly a week's worth of food within its four walls at any given time. However, remember that this is a week's worth of food in normal circumstances. As the 2020 toilet paper situation showed, once people fear they can't get more of something, they stock up. Therefore, a week's worth of food at the grocery store can be gone in a matter of days or hours.