Wild guess,the full length resize is giving the clearance for your round to find its own center as it expands.
A common misconception.
All bottleneck cases headspacing on their shoulder perfectly center their shoulder in the chamber shoulder when the firing pin drives them there before the primer detonates. In many rifles, case shoulder's well centered in the chamber shoulder as soon as the bolt closes. Doesn't matter how the case is sized; even new cases do this.