This isn’t about imperial vs metric, it’s about measuring by mass vs volume. A good example here is flour. Weighing out 30 grams (or about 1 ounce) of flour will always result in the same amount. On the other hand, you can densely pack flour into a 1/4 cup measuring cup, you can gingerly spoon it in little by little, or you can scoop and level. When you do this you’ll get three different amounts of flour, even though they all fill that 1/4 cup. Good luck consistently measuring from scoop to scoop even if you use the same method for each scoop.