I'm pretty sure that one cup of different foods weigh different amounts, because there are 2 different measures involved.

A cup can be thought of more like a displacement measure, i.e. it is a measurement of a pre-defined "space" amount. I cup of four, butter, sugar, take up the same amount of "space"

But some things are more dense than others, so they weigh more, hence flour being lighter in grams than sugar.

I've tended to switch over to calculating my food amounts all in grams, weighing everything, than figuring out the nutritional values from the number of grams. It really helps that the nutrion counter here at this site offers values in both 100 gram units, and cups, teaspoons, etc.

For example...how much does a cup of apple weigh. Well. it depends on how small you cut the pieces and how tightly you stuff them into a measuring cup. So, for a better measure, weight the apple pieces. I had half an apple this weekend. It weighed 83 grams. The other half weighed 86 grams ( I didn't cut it right down the middle, and it wasn't perfectly round.

Hope this helps...