Getting milk in one gallon jugs and infrequent visits to the grocery store mean timing your consumption to coincide with a trip worth the time, so it took awhile to counter the milk jug illusion which tends to throw off scale visual levels for judgement.
These jugs(also called milk gallons) have pseudo-conical top halfs, so when pouring out milk, that half seems to empty faster, instructing you, albeit falsely, to slow down consumption by say one-eights so as not to make a specific visit to replace it. A couple of days later, the level reaches the cubical half, and then the level hardly seems to dip, panicking you to believe you will still have milk left when you get a new jug.
It's even worse when the level is so low when viewed from outside that what one might assume it will last for just one day actually lasts three times.
In essence, the milk jug empties out faster when opened and doesn't want to empty in the end.