In any chemical reaction that produces energy, it is the mass that is converted to energy, i.e. the quantity of molecules available for conversion and the conversion efficiency (incomplete vs complete combustion). We use weight to determine mass, which is reasonably accurate for most situations. Assuming powder is hydroscopic (absorbs moisture), what are we talking about for % of weight? 5% maybe? Probably not that much actually. How much does the humidity vary at any given time? From winter to summer in MN, it's a pretty big change, but that doesn't mean that the amount of moisture the powder will absorb and release is directly related to humidity 1 for 1. In other words, a 50% atmospheric change may only be 25% of 5% (or 1.25%) change in weight in a non-condensing environment. (The implied warning here is to allow the container to reach ambient temperature in the reloading environment before opening the container and dispensing any powder). In any kind of controlled environment (air conditioned and heated), I'd be very surprised if the change in weight due to water absorption even approached 0.5%. The density of a volume of powder, on the other hand, is the real issue, as discussed earlier, and that can vary significantly.
I might also add that more attention should be given to conversion efficiency, which is influenced by the type of powder chosen for the application, volume of case, crimp, distance to lands, and all those other factors that we think of as affecting accuracy, but really also relate to the burning of the powder and the rate at which pressure is generated.