The way the headset tightens is the split centering cone is pushes the top bearing down shrinks into into is until it's wedged between the bearing and steerer forming a tight press fit. Headsets vary somewhat but the principle is the same. If whatever presses on the cone bottoms against anything other than the cone, you won't get a solid fit between the steerer and bearing.
Building up the steerer diameter a bit raises the height at which the cone jams and solves the problem (if that's what it is). The problem can also be solved with a spacer with an OD small enough to press the cone without touching on the OD.
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