Thank you John and Jan.
The way we tested was 20 slicing cuts under 2 kg load in the same line on the board, move the edge to a fresh board surface, do another 20 slices and so on.
Both your explanations are exactly what must be happening - burnishing of the metal off the sides of the edge near the apex inside the tiny grooves, and smoothing off the irregularities as Jan has nicely described.
I will give myself more time to think it over and will add a paragraph to the Conclusions explaining what we see in the experiments.