Its like anything stacked one upon another. The more powder you have sitting on top of the powder at the bottom, it will pack the charge tighter in the charging chamber receptacle. Look at your dillon powder reservoir, see that V shaped baffle at the bottom, That is there to allow only a retained weight of powder press down on the powder that just dropped into your charging cavity. All powder has a degree of "fluff" since the particles have air all around them. But you can compress some of that air out of it with a large volume of powder pushing down on the powder at the bottom. Its the job of a auto measure to give a consistent weight of powder by metering its volume, not its weight. Putting Fluff in check consistently is the goal. Thats why REALLY meticulous reloaders use a trickle charger for each load. With one of those, there is no fluff variable in your dropped charge. Its all done directly onto a scale buy dribbling a few particles of powder at a time.
Thats why I thought it interesting to see that powder measure with a hammer built in, It would some how settle some of the fluff out of the powder to be dropped into the metering cavity.