Centering question
 

I'm not huge on grading, but I've been wondering -- on issues that don't have borders, how do PSA, BGS, SGC, etc determine the implied centering of a card? Who's to say what the centering is "supposed" to look like if it's full bleed?

They use other elements like the name, designs, team, etc. to determine what the centering should be. However, they do seem to be more inconsistent on full bleed cards.

I assumed that would be the case but I guess the larger question is how do they know which placement of those elements is correct? Like if on some 1993 SP Derek Jeter cards the logo is 5/16 of an inch from the edge and on others it’s 3/8 of an inch from the edge Who’s to say which is “correct”?

They make an assessment (eye test, mainly) and go with it, as far as I've heard. They may be able to identify better based on other cards from the same set.
If you want me to tell you the grading companies know what they're doing, wait for someone else to answer... ;-) I really doubt they go back to the manufacturer and ask what the centering was intended to be measured as.

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Why would this be an example? It has defined borders to determine centering.

My bad, read too quickly and missed the borderless piece.