For this site, 120 grams is exactly one US cup of all-purpose flour when it is fluffed, spooned and leveled. The same 120 grams is only about 0.61 cups of granulated sugar and about 0.53 cups of butter.
The one-cup flour relationship applies to the measurement method shown here. Scooping flour directly from a bag can compress more flour into the cup and break that relationship.
This amount is especially helpful when converting American flour measurements to weights. It can also appear in cake layers, muffins and small loaves where flour measurement has a noticeable effect on texture.