Why cups-to-grams depends on the ingredient
A cup is a volume; a gram is a weight. The bridge between them is density, and every ingredient has its own: a cup of all-purpose flour weighs about 120 g, a cup of granulated sugar about 200 g, and a cup of honey about 340 g. Generic unit converters that use water's density will get every one of these wrong.
Flour is the troublemaker. Scooping directly from the bag compacts it — you can pack 150 g into a "120 g" cup without trying. If you must measure flour by volume, fluff it, spoon it into the cup, and level with a knife. Better yet, use a kitchen scale: it's the single cheapest upgrade to your baking accuracy.
Also note that a US cup (240 ml) differs from an Australian cup (250 ml) and a Japanese cup (200 ml) — one more reason weight beats volume for recipes that travel.