Did you run a full container of powder through the PPM, as instructions say, before using it to measure charges? THis is necessary to put graphite on all the internal sliding parts and ensures more accurate results.
When I fill my measure, I tap the container 10 times, every time, for consistent density, thus consistent results.
Do you use a consistent smoooth stroke to dispense the powder? Always use a full stroke up to fill the measures volume chamber, a momentary pause, then a full stroke down, including a momentary pause at the bottom, to dispense the powder. Don't rush up or down.
I find that is the most accurate way for me. Usually less than .1 grain off. I count my variables in granules of powder.