I have been using Nobel Sport Prima V for minor power factor 9mm loads, and noticed some significant variation in charge weight (up to 1 grain in a 3.5 gr charge). At first, I thought it was just that the powder didn't meter well in the Dillon powder measure. But that wasn't the case. Charges were very consistent for 100's of rounds, but fluctuated up and down steadily.
Inspection of the powder hopper revealed stratification of grain sizes, with the largest grains on top. This is a recently discovered phenomenon in the physics of granular media called the Brazil Nut Effect. Counterintuitively, the largest, heaviest particles end up on top. This is why rocks keep sprouting in plowed fields. Of course, powders with a very uniform grain size (e.g. Titegroup) wouldn't have this problem.
My solution is to mix the powder with a chopstick when it's half used, and then again when I fill it. I also mix the powder in the jug before pouring. My charge weights have been spot on so far.