Watch it, bud! I've MADE Vernier scales (well, OK, one; for my lathe's tool post scale).
Below is a very exaggerated illustration I cooked up awhile back that shows what Bart described about the case growing as the sizing die starts to narrow it. The brass has to flow somewhere. It also shows why most neck growth occurs during sizing.
Many folks have good luck just turning the sizing die to get the desired shoulder setback, but a press that is light and flexes much or a die that's not solidly in place can allow some variation in results when two cases don't require the same seating effort (getting a range foundling or mixed history brass into the group, or uneven case lubing can do this). In those instances using the Redding Competition shell holders or a shim on top of the shell holder so there is some over compression of the die against the shell holder at the desired case size, and that can also help make the result more uniform.
When the unevenness is due to springback being greater in one case than another, I've found that a case that comes out too long can be run back into the sizing die and held there a few seconds while it relaxes. Often one or two times will shorten it another 0.002". This can be useful with your first time putting range foundlings and once-fired brass through it.