The cup may be accurate, but the filling method was not consistent. Dry measuring cups are intended for ingredients such as flour, sugar, rice, grains, and powdered additives, yet each material responds differently to scooping, pouring, tapping, and packing.
A reliable procedure therefore matters as much as the container. If a recipe specifies a level cup, fill it without deliberate compression, then remove the excess with a straight edge held across the rim. If it specifies packed brown sugar, press the sugar firmly enough to hold its shape and follow that method every time.
