It isn’t something I can prove, but I suspect sometimes there can be something included in a sterling alloy that really shouldn’t be there. Sterling must contain at least 92.5% silver. The rest, or most of the rest, is assumed to be copper, but that is undefined and can be anything.
When silver was $4.70/ounce (sigh) I bought a lot from Swest, and some went unused for a very long time. When I finally got around to using that, it was a very dull gray. Sanding didn’t help so I sawed through a piece and found it was dull gray all the way through. This was just from the sterling ‘hanging around’ a number of years. Pickling did not help. The sterling just turned a dull gray and that was that.
When the price of silver went to over $40/ounce I sent it all in to be refined and did very well so I’m not complaining! But the only explanation I can think of for graying like that, when fire stain is not a possibility, and which pickling can’t clean up, is something being in the alloy other than silver and copper that is causing the gray.