Different brands of Black Powder with the same granulation (something like FFg) and the same volume will weigh differently. I had that problem when I switched from Schuetzen to GOEX. I tried weighing samples of the two powders so that they were equal in weight, When I put them in the casing (.38-40), the GOEX didn't quite fill up the casing as much as the Schuetzen did. Had to add a tad more GOEX. Not much, but just a bit.
At least that was my experience.