Although it sounds strange that a cup converts to fl oz or ml, this is actually correct!
A cup is a volumetric measurement whereas units such as grams are a weighted measurement, this means it's very dependent on how loosely or packed the cup is to accurately convert to a weighted measurement such as grams.
This is why often dry ingredients measured in cups will convert to ml as they are both measured in volume, not in weight.
We do not convert weighted ingredients from Cups to grams as these change vastly between the ingredient and the preparation method (grated, cut, sticks, loosely packed, packed, etc.).