It's not always a purposeful grift when that happens.

When dealing with an actual different sku like that, they don't necessarily get the products at the same time, and they don't sell through them at the same rate. It could be that the smaller boxes have been sitting around longer and the price was lowered to sell through before their expiry dates.

They don't always put sales tags on everything that's discounted a small amount, especially if it's expected to be longer than a few days.

Especially if it's a large company that likely has automated basic things like that to update pricing for stagnant inventory in a specified way.

Just don't take it extreme like my ex roommate. He would buy bread by picking every brand and type in the bread aisle, count how many slices were in the loaf, cheapest bread in price per slice he would buy.