When going by weight, it doesn't matter if you sift before our after you weigh it. When going by cups, you're supposed to spoon it into the cup and then after measuring, sift. When it says "1 c. Powdered Sugar", you sift after, when it says "1 c. sifted Powdered Sugar", you sift first. When you sift, it incorperates air into it. The air changes the volume but not the weight. When a one pound box doesn't measure out to 4 c. it's because it has settled and the air has gone out of it. If you sift and then measure and have some left over it's because of the extra air you put into it. You don't have extra product, you just used less because the extra air you sifted into the product took up space in your measuring cup. I hope I explained that clearly.