Powder Position
Where the powder charge sits inside a partly filled case, forward against the bullet or back against the primer, which can shift ignition and velocity.
When a charge does not fill the case, the loose powder can settle either forward against the bullet base or rearward against the primer, depending on how the rifle was last handled. That difference in position changes how the primer flame reaches the column, and with some powders it produces a measurable swing in velocity from shot to shot. The effect is most pronounced in large cases running reduced loads, where case capacity far exceeds the volume of powder present.
Shooters who chase low extreme spread often tilt the muzzle up before each shot to settle the powder consistently against the primer, removing one source of variation. A higher load density that nearly fills the case is the cleaner fix, since a full case leaves the powder no room to shift. Slower-burning extruded powders tend to be more position sensitive than dense ball powders that pack tightly to begin with.