Manufacturers often use big molds for these containers because they take into consideration the plastic shrinkage that happens during production. Because of this extra allocation, the jar doesn’t appear full even when there’s a complete gallon of milk inside. To make the packaging acceptable to consumers, they adjust how the volume looks by placing a circular dent on the side.

Other articles point out that these concaved parts have other purposes. When the milk gets old, it produces bacteria, which causes the dent to expand. The crafty design is also a form of structural support. When the jug drops and pressure inside spikes, or when milk expands as it freezes, the dimple will swell to prevent the plastic from breaking.