It's not a headstock issue. It wasn't cut on my machine and I can promise you, it wasn't the headstock. The phenomenon has been described but I hacked together a visual.
Red line through the middle is one opposite set of flues on a 6 flute reamer.
One flute hits point A to make a cut. The ramp in the flute, coupled with an unsupported oppsite side (B) can flex/push/force the reamer into the B section. The further along you get, the more gets eaten away where B is, making it easier to push the cutting further into B. Another impact to the mess, is the ramped nature of the lead. You're pushing the reamer into and up that ramp... more force on the unsupported side.
On a standard 4/6 land barrel, the reamer is always equally cutting and supported on opposite sides.
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