Pyrex glass measuring cups are for liquids.
Plastic and metal "cups" are for dry ingredients.
It's an old rule of thumb that no one seems to remember...
The issue is a difference in volume.
One half gallon of milk weighs 4#s, contains 8 cups (8x8oz = 64oz/8 = 4#s).
Four pounds (#s) of sugar also contains 64oz.
Weights can be equal, but the mass (or ...