Actually , you have a very good an logical question mark .
Lets see if I can attempt to explain it to you .
See BP, unlike smokless , starts out as one BIG grain . Its then crushed and sifted through screens to be sized . There is no consistent size or shape . Again unlike smokes which has a distinct size shape and weight to each grain .
So lets say your weighing out 2 F and its showing that 100 grains weighs about the same as 100 grains by volume .
Now replace that 2 F with 3 F , 4F or 1 F .
Now what you will find is that by volume , they all weigh in at the same weight.
BUT here is the kicker . That weight., volume to volume or weight to weight will produce higher pressure because the burn rate is faster even though it all came from the same original brick of powder .
4F will produce un real pressures at 100 grains . While 1F at the same volume will produce less pressure then 2 F
The other thing that happens is the mix itself weighs differently .
See with smokeless , a given powder , be it red dot or what have you , has a specific chemical make up that dictates its weight .
Black powder is not that way because of the charcoal . See different powder use different woods to make charcoal / swiss uses a given hardwood . Goex a different one and elephant and kick , still different ones . These woods , by their make up have different weights and densities . As such the powder can weigh more and in fact in some cases like with swiss , actually produce high pressures then the same weight an volume as goex . Synthetic are not this way as they are made with chemicals much like smokeless is made .
Now that all being said . Some folks do weigh each charge . These folks are normally long range tournament shooters must have every load exactly the same . However the ones I know do not use the same process as you would use for say reloading cartridges . What they do is once they find a volume load that works well . They then compress the powder and re figure the volume . They then weigh that volume and record it .
Then on the next shot they do the same thing again so as to insure that everything is exactly the same as before . Basically they do everything in their power to ensure absolute minute to minute consistence . So whats left is to control their own human inconsistency
But even then these folks are for the most part starting out with a volume of powder so as to define the weight
I would also agree with the others here . Unless your going to carry pre measured charges . There is no reason to go through the hassle of weighing charges unless you shooting for a string at 40 rods or better