It's difficult to be sure without a photo, but probably it was part of the program "precios cuidados". It was not 100% mandatory, and not 100% voluntary, it was something in between, so I don't blame you for being confused. The factory and the supermarket got some benefit in exchange of keeping some of the products in the list at a low price. For example, nobody fussed them if the premium brand increase the price.

Something similar happened with the Big Mac in McDonald's. At some point the price of the Big Mac was like 30% lower than the other similar hamburgers. Inside the restaurant, you had big advertisements of the other combos, hamburgers and premium hamburgers, but the Big Mac appeared only in the small list of prices in the corner. I found this link with more details https://www.pri.org/stories/2012-02-07/argentina-likely-mani...