I am making a spreadsheet to convert volume recipes into weight for my
cake research. Most of my volume measurements have been consistent with
listed weights (water, sugar, oil, eggs), but I am finding inaccurate
results with flour and cocoa.

While I find the standard weight (listed on a number of web pages, and
on the box itself) of cocoa powder (both Nestle and Hershey) to be 80
grams per cup, when I weigh a cup I always end up with 65 grams.
Likewise, my gluten-free flour mix (Namaste) is listed (on the bag) as
being 138 grams per cup, but when I weigh a cup it comes out to be 120
grams.

I should note that I am measuring the cocoa powder and flour by spooning
it into the measuring cup (the same one I used for the ingredients that
match up with their expected weights) and leveling it off with a
straight surface (the back of a knife). I should also mention that I am
not using a sifter, but I wouldn't think that would have such an effect
on the weight (18% on the cocoa powder and 13 percent on the flour).

Any ideas on what is going on here? Should I trust the listed weights
or the weights I am coming up with on my own?

Jon