I think it has to do with whatever coating they use on the dies to get a burnished finish. I had matching series of specks show up on differnt coins, which would have to be atrributed to the die. Also one coin would show less specks than the other, so I think it is a type of deteriation as the die wears out. Most of these I was only able to detect under a USB microscope, with only one or two of the specks showing in hand. My Grand Canyon look nice in hand but I'll put it under the scope to check.
I wish these were more available to the public. I hate how the mint does business with other dealers on certain low mintage coins. Like these and the eagles. It prevents me from wanting to collect them.
The bullion versions of these coins are only available to be purchased by “authorized purchasers,” and with that fact I can agree that the Mint handled the issuance of these poorly. However, the Mint does allow the collector versions of these coins to be purchased directly from them for $279.95 each. The picture above your post is of a collector version.