[Brian Pfleuger]

That seems like a virtual impossibility if you're using lock rings on the dies.

First, how much "movement" are you getting? It's not unusual to have a couple thousandths difference in bullet length.

Beyond that, I suspect you're maybe not using consistent strokes on the lever?

Every seating die I have (all Lee) in both handgun and rifle has pretty stiff resistance on adjustments to the seating stem. I can't see how it could possibly move by accident, let alone in a couple of strokes.