But the shelves of Grocery stores are different: you cannot just change things around all the time without your customers complaining or going to another market with better offers. The amount of shelves is more or less constant throughout the life of a store and promoting your own brand often means you lose out on the money others offer you to get a good spot.

On Amazon there are as many shelves as there are product categories and the thing has much much more fluctuation.

What that means is, that a "promoted" product is sometimes the only one which stays at a fixed spot, while all all the others are to be found on the miscellaneous-pile.