In a perfect world the third party grading companies should be evaluating cards objectively, and not letting any sort of biases creep into their final decision for a card. Yet, we all know that we've seen some questionable grading decisions in the past, based on the numerous trimmed cards that have ended up getting numerical grades.
A recent video posted by Vintage Card Curator on YouTube challenged this fact by taking a look at what they refer to as the "9:10 ratio"; a simple calculation that looks at the number of PSA 9 cards and divides the number by the number of PSA 10 grades. A simple way to interpret the number quickly--if for example we see a card has a 9:10 ratio of 12:1 it means that a card gets a 10 grade for every 12 cards graded a 9 by PSA.
The focus of the video which I've posted below is on mostly modern era cards (from 1978-1993), such as the ever-popular 1993 SP Derek Jeter card.