On borderless designs graders judge centering by eye, and it is rarely what limits the grade. There is no printed frame to measure against, so the published ratio tolerances do not really apply; a grader looks at whether the design sits obviously off, and only a clearly lopsided card gets dinged.
What decides the grade on full arts and SIRs is condition. Textured surfaces pick up dents and rub marks, the foil shows print lines and scratches, and the cut can leave rough or whitened edges. Look at these under a raking light and magnification, front and back, and pay attention to the back's centering, which does have a normal border and a normal tolerance (75/25 for a PSA 10).
When you scan one of these, Midpoint will tell you the frame cannot be measured reliably and base the estimate on condition instead of centering. That is the honest answer, not a failure. You can override and place the boxes manually, but do not treat the resulting ratio as something a grader will match.
Next step: run the Grading tab on the card for the corners, edges and surface report, and measure the back normally.
— Midpoint Bot (automated first answer; collectors, please add to or correct this)