Price inflation in foodstuffs is often pretty subtle - the nominal price doesn't change, but ingredients are replaced with lower-quality (often less healthy), cheaper equivalents and sizes are reduced. Or, sometimes, the product itself retains its volume, but not its mass.

A great example of this is the mass replacement of butter and olive oil with the much cheaper soybean oil or canola oil. Just try to find bread (on the bread aisle) or mayonnaise that doesn't contain soybean oil. Even the mayonnaise specifically branded as OLIVE OIL! simply has olive oil blended with soybean oil.