The path through a supermarket is designed, not accidental. Layout decisions affect what ends up in a cart at least as much as the prices printed on the shelf tags.
Staples are placed to lengthen the trip
Frequently bought essentials are typically separated from each other and from the entrance, so that collecting a short list requires walking past a long series of other categories.
Exposure is the point. A shopper who passes more categories buys from more categories, and each additional department contributes to the total.
This is why milk, bread and eggs rarely sit together near the door, even though nearly every trip involves at least one of them.
Shelf height sorts products by margin
Positions at adult eye level and within easy reach are the most productive shelf real estate, and they are allocated rather than assigned by chance.
Higher-margin lines and brands with supplier support tend to occupy those positions, while value lines are placed at the extremes of the bay where reaching requires effort.
The consequence is that comparing only the products directly in front of you compares a filtered selection. The full price range usually requires looking up and down.
Perimeter and center behave differently
Fresh departments occupy the perimeter because they need utilities, generate waste and benefit from a route that circles the store. That route passes every aisle entrance.
The center holds shelf-stable goods, which are more price-comparable and more often the subject of a deliberate choice. Promotions there compete against direct alternatives.
A trip confined to the perimeter therefore has a very different price profile from one that works the aisles, independent of what is bought.
Displays interrupt the comparison
End caps and floor stacks remove an item from its category, which removes the alternatives that would otherwise sit beside it.
An item on display reads as promoted even when the shelf price has not changed, because position carries the same signal a discount does.
Checking the item's home shelf before adding it is the simplest correction, and it is the one displays are designed to discourage.
Checkout is the last placement decision
The queue is a captive area with a known dwell time, which makes it suited to small, high-margin impulse goods that need no consideration.
Self-checkout areas reduce that opportunity, which is one reason store designs have shifted the impulse assortment toward the approach rather than the lane itself.
Understanding the layout does not defeat it, but it does explain why a basket grows on a trip that started with three items in mind.