It's assumed that all jugs from a mould hold the same volume, but not every mould holds the same volume, so by aiming high for the initial design of the mould and then adding spacers to the side of the mould as needed you can achieve a modified mould that stores the correct amount.
DISCLAIMER: I am not a milk jug engineer, that's just how I interpreted it.
Nice, but I really hope they eventually address the issue of plastic production in the first place.