Some products in a supermarket are ordered every day and others every few weeks. The frequency is calculated per item, and it explains most of what shoppers notice about availability.
Shelf life sets the outer limit
Fresh bread, salad and prepared meals have a usable life measured in days, so ordering them weekly would guarantee that most of the delivery was discarded.
Dry goods and tinned products last for months, which allows infrequent ordering in larger quantities and cheaper transport per unit.
Chilled products sit between the two, and their ordering frequency is the most sensitive part of the system because both errors are expensive.
Shelf space is the binding constraint
A store can only hold what fits on the shelf plus a small back-room reserve, and modern formats have reduced storage space in favour of selling area.
An item with high sales and a small facing must be replenished frequently regardless of its shelf life, simply because the shelf empties within a day.
This is why a popular product can be out of stock while an obscure one beside it is fully faced. The gap reflects turnover against space, not popularity alone.
Forecasting drives the order
Ordering is largely automated, with systems projecting demand from sales history and adjusting for the day of the week, local events and weather.
Weather matters more than most shoppers expect, since a warm weekend shifts demand towards salad, drinks and things cooked outdoors within a single forecast cycle.
When the forecast is wrong, the error appears about a day later on the shelf, which is why a sudden change in conditions produces visible gaps.
Promotions distort the pattern
A promoted product can sell several times its normal volume, and the store must hold enough to cover the whole promotional period from a limited amount of space.
Buying and shelf planning for a promotion are agreed weeks ahead, so a promotion that performs far above expectation cannot easily be resupplied mid-week.
Demand also falls after a promotion, since customers have stocked up. The system must anticipate that dip or it will over-order into a week with no sales.
Waste has become a managed number
Unsold fresh food is a direct loss, and stores manage it with markdowns at defined times of day and with redistribution arrangements for surplus.
The markdown schedule is itself calculated, since discounting too early loses full-price sales and too late leaves stock unsold at closing time.
Reducing waste and maintaining availability pull in opposite directions, and where a chain sets that balance is visible on its shelves at the end of a trading day.