That's a good question Calvin.
The barrel tubes had been rough bored, prior to the choke reaming. The term 'rough' didn't suggest it was a coarse cut, or unsmooth surface, it merely differentiated the operation from a final polish.
The bore was then given an operation, taking it down to 'near-bore' dimension (varied from early guns of slightly over .750" to late guns closer to today's .729") The last few inches were finished with the choke reamer.
The flights (or flutes) of the reamer wanted to resist cutting into the barrel wall, and naturally sought to move off the wall, toward center. As all the other flights were doing the same thing; seeking a balance in concentricity, the reamer stayed in the geometric center of the tube.
The Sunnen hone, much like a brake cylinder hone, except on a long shaft, with a hollow center allowing cutting oil to flood the area, also stayed centralized in the same manner.
Variation in barrel wall measurements are almost always a result of 'striking' the outside surface of the barrel; the operation of prepping the barrel for final finish.
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