Highest Known... Huh?

There was a recent big auction catalog mailed out (the one with the PSA 10 Goudey Gehrig), and throughout the entire catalog it would say things like "Highest PSA known". What does that mean? Are there PSA graded cards out there that are unknown?
That's completely confusing what they mean by that. Are they saying that THEY simply don't know of any higher, or that there are PSA graded cards out there in which NOBODY knows about?
That's completely confusing what they mean by that. Are they saying that THEY simply don't know of any higher, or that there are PSA graded cards out there in which NOBODY knows about?
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Seeking primarily PSA graded pre-war "type" cards
My PSA Registry Sets
34 Goudey, 75 Topps Mini, Hall of Fame Complete Set, 1985 Topps Tiffany, Hall of Fame Players Complete Set