Two packages of the same product routinely hold different quantities, which makes their headline prices incomparable. Unit pricing exists to resolve that, and it frequently contradicts intuition.
Package sizes drift over time
Manufacturers adjust pack contents in response to input costs, often reducing quantity rather than raising the price because the price is what shoppers notice.
The pack looks similar, the shelf price holds steady, and the cost per measure rises without any visible change. This happens gradually enough that the shift is difficult to observe.
Unit pricing catches it immediately, because it measures what is inside rather than what the label costs. The shelf figure is the only place in the store where a quiet reduction in pack size becomes visible at a glance.
Larger is not reliably cheaper
The assumption that bigger packs cost less per unit holds often but not always, and promotions are the usual reason it breaks.
A promoted medium pack can undercut a full-price large one, and multibuy offers can beat both or neither depending on the terms attached.
Because promotions rotate weekly, the cheapest size for a given product changes constantly, which makes any fixed rule unreliable.
Comparison requires a common measure
Unit prices are only comparable when expressed in the same measure, and stores sometimes show one product per hundred grams and a neighbouring one per kilogram.
Products sold by count against products sold by weight create the same problem, particularly where the individual items vary in size.
Rules on how unit prices must be displayed vary by jurisdiction and change over time, so the presentation is not consistent everywhere.
Concentrates and dilutions distort the figure
A concentrated product carries a higher unit price while producing more finished result, so the shelf comparison understates its value considerably.
The same applies to items where a portion of the weight is packaging, liquid or bone, none of which the household actually uses.
Comparing cost per portion rather than cost per package weight corrects this, though it requires knowing roughly how far each option goes.
Where the habit pays off most
The benefit concentrates in frequently bought staples, where a small difference per unit compounds across every week of the year.
For occasional purchases the effort rarely justifies itself, and the time is better spent on the items that repeat.
Checking unit prices on the dozen products a household buys constantly, then leaving the rest alone, captures nearly all of the available benefit for very little effort. Once those comparisons are known they hold for months, and only a promotion or a pack change disturbs them.