Answer
According to available estimates, food prices in Uruguay may remain roughly the same even over several years: it has been noted separately that they changed very little over a three-year period. This does not mean that every product always costs the same amount.
The amount on a particular shopping receipt is affected by the store, neighbourhood, season, promotions, package weight and chosen brand. Fresh fruit and vegetables depend on the season, and prices and selection may vary between branches of a chain, so a single purchase is a poor reflection of the entire market.
In practice, this means you can base your budget on the general price level, but not treat a single receipt as a fixed benchmark. It is better to keep a short list of regular purchases and compare it periodically across several places: this will reveal both real changes and temporary discounts.