Can't slab--only one known?

[Hankphenom]

At the Philly show, I ran into Earl from SGC. He told me that Legendary had sent them a Walter Johnson card they had never seen before. I said, "That's my card!" to which he replied, "I don't think we're going to be able to slab it, because there's no exemplar."

So let me get this straight. They can never slab a card that hasn't surfaced before, so no one-of-a-kind card can ever be slabbed? Therefore, the second example to surface will get slabbed, but not the first. Now I'm even wondering if they even keep a picture of my card, in case another one does ever show up.

Seems to me there should be enough evidence of age, wear, materials, ink, style of printing, etc, to give them enough confidence to slab this card, but I guess there must be some legal reasons for them not to.