Maybe terminology has us perplexed, here.
It's relatively unlikely that there'd be pitting under the finish of a gun that was polycoated at the factory. Pitting is generally a function of rust...
Its also unlikely that the finish could have developed pitting duirng a refinishing process. That happens too quickly, and a refinisher of established quality, like Robar, would not let something like that leave their shop. (Some charge extra for hand work to remove such surface imperfections.)
Maybe its not pitting?
Could it simply be imperfections in the metal, covered by the original parkerizing and polycoat? Polycoated guns aren't given the same "finish" attention at the factory as are guns that are blued...