So, if I've got this right, it's an EQUIVALENT volume compared to what real black powder would weigh. Not the same mass, but the same volume.
And just to add to the confusion, the shotgun shooters often specify their powder charges by "ounces", again not literally. When they say "I shoot 1 1/4 ounce shot and 1 1/4 ounce of powder", what they mean is that they use a shot dipper set to measure 1 1/4 ounces of lead shot to measure their powder charge. A lot of them don't even know their powder charge in grains.
But then, the ml shotgun crowd has always been a little touched in the head