“Any explanation why the bluing on the 1906 proof marks seems to come off?”

When the PW proof mark die is struck into the metal it forms a plastic deformation, meaning the metal is displaced and doesn’t return to how it was before. If you examine a Winchester proof mark under high magnification you will see that the little tiny waves of metal radiate out around the impression itself for some small distance say 1/16″ plus or minus. This is similar to a rock dropped into water. There is the big splash hole at the center but little waves radiate out around it. Winchester gun metal has plastic qualities too and acts the same only on smaller scale.

Since the metal surface is deformed the thin layer of metal turned blue on it is deformed as well and a halo of white metal is formed in and around the proof mark. The exact molecular process of why the blue turns white and goes away is over my pay grade. But it happens because the metal was deformed.