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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PWCC: The Fish Stinks From the Head
PSA: Regularly Get Cheated
BGS: Can't detect trimming on modern
SGC: Closed auto authentication business
JSA: Approved same T206 Autos before SGC
Oh, what a difference a year makes.
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