[jchcollins]

The biggest reason is the higher the grade, the less the tolerance for centering problems. A 9 is supposed to no worse than 60/40, but if the card is only a 5 otherwise, centering can be as bad as 85/15. Centering is on a sliding scale, which often confuses / frustrates those new to grading.

The other thing to point out as others already have of course, is that they often eyeball centering and can be wildly inconsistent. Back in the early days of TPG's in the 1990's and early 2000's, centering as the number one main important paranoia-driven criteria of grading was not really a thing yet. Centering has become much more important in determining overall grades for PSA and SGC even just within the last 10 years. SGC just this year it seems has really gone overboard. I've seen cards they would have easily called NM a decade ago get knocked down to 5's just because of centering. A lot of it is really borderline silly...