Being as the outside dimensions are the same (or as close as we can get them) there are only two things that will affect the weight of the case.
Thickness of the brass, and its density. Cartridge brass is not 100% identical maker to maker and there are also variances lot to lot.
Given its the same outside the heavier case must either be thicker (which affects volume) or made of a more dense material (which may not affect volume). Could even be both together.
We can do very precise measurements many ways, and they are useful for showing general trends, and the specifics of what was measured.
HOWEVER, there is no guarantee that the brass in your hands will be identical to what was tested & measured. SO, don't take the fine details as holy writ.
Your stuff should be very similar. It might even line up perfectly with someone else's test data. But don't assume that, do your own measurement on what you have. That's the only real values you need to be concerned with.