Another theory: An initial undersized hole was drilled using any of the methods they suggest which produce no concentric rings. Then a single point boring tool

Another theory: An initial undersized hole was drilled using any of the methods they suggest which produce no concentric rings. Then a single point boring tool was used to machine the hole to its final dimension. The hole tapers 1 cm over its length. 0.5 cm of wear of the single point tool in use would account for the decrease in diameter.