Stick a couple used Cling-Free dryer sheets where they touch the powder despenser. That should disperse most static charges.I have found that if I change discs on mine, I do get variations for over a dozen rounds. If I am loading 9mm, I generally use a 38 case and cycle the powder dispenser several times, checking the weight of each load until they settle down. I think it has something to do with the static charges on the powder and plastic parts.
I just use the longer 38 case as it is the same diameter as a 9 but is longer and easier to hold. I lower the arm so that the shell plate comes up and start checking loads I run the lever by hand.
Someone mentioned that it is important to have the arm pull the powder disk all the way back. I think that is really important to have the spring on the return chain compress when you have the arm all the way up. If there is variation there you will have variation in your loads.