Do you have a baffle in it? What type of powder... ball, flake, stick? How long is the drop tube? Are you "cutting" powder at the throw end? Is the bottle full and do you top it off as you go? What is the humidity in your area? Some or none may be applicable.
Perhaps you need to develop a more consistent "throw" when moving the lever. The speed of movement and the upper and lower "taps" should be as repeatable as possible.
1/10th a grain variation (or less) should be the norm... not, according to your numbers, as much as 4/10ths.
Shoulder Drive Nicholson Club