Why does a plastic shortening like margarine weigh less per cup than a melted one or than a cooking oil?

Solid butter or margarine might have air in it, so the amount you get into a cup is somewhat less than when you melt it and it compresses. This means you have more in a cup and it will weigh more. Always check if a recipe calls for butter or margarine to be melted before you measure it or you may have the wrong amount.