Ahhh! That helps quite a lot. I don't think this is a cast forgery, so the fake would pass the "ping" test. I have seen this effect on outlines (the ear of wheat is typical) before. The explanation I have heard is that there is a computer-guided (probably European, I have not seen such copies of US coins yet) machine that can make pretty good 3 dimensional copies. The salesmen use copying a (usually common) coin as a demo and present the copy to the prospect, without bothering to invalidate it. It may be a fluff story, but it does fit with the known facts. The only copy of this kind I have handled did not have the right weight.
Peter