The same yoghurt costs a different amount in every chain, and the packs are never the same size. Put two shelf prices side by side and you have compared nothing. The Per-Kilo Leviathan is that mismatch: it surfaces whenever a promotion makes the smaller pack look like the bargain.
So every price in LowPrice carries a unit price. A product knows its pack size and its unit, kilo or liter, and the app derives the price per kilo from them, the same way on the product page, on a catalog card and in search results. A sale shows its own badge and the date the promotion ends, because a cheap price that ends tomorrow is a different kind of price.
The rounding rule matters more than it looks. Unit prices are compared at full precision and rounded only when the text is drawn, otherwise two chains that differ by a fraction of a coin appear to tie. The comparison itself is just a sort, and the chain that is cheapest per kilo, not per pack, gets the first row.