Actual blackpowder is pretty much volume =weight.
However there are substitutes & so on that both have a different density, & some even need a slightly reduced charge to run safely.
Now if this was just a couple of percent, it'd be no big deal, but if you measure, for example 50 Gr by volume of real FFg, & then the same volume of Pyrodex, or Jim Shockey's the difference when weighed will be about 60%. Thats a significant change, even with the more forgiving black powder as opposed to modern propellants. Now add the suggested 15% reduction for triple-7 & combine them & you have the potential for a 56% overload!
Some kind of standard was needed & volume was what was chosen. I guess you could remove confusion by saying its the volume equivalent of a weighed FFg charge.
Allan Quartermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”