Here's another link to the Henderson video:
https://www.youtube.com/watch?v=6wTMS1dmddc&t=216s
If it doesn't work and you care to review the video, simply go to Youtube and search "Vintage Card Curator Henderson." The video is approximately 14 minutes long. He goes through several different analysis to show the statistical improbability of 9s to 10s of the Henderson card (and 10s in general) relative to the rest of the set. At the time the video was made there were over 1,900 PSA 9 Hendersons and only 24 PSA 10s. The ratio of 9s to 10s is 81 to 1! For the rest of the set, the ratio of 9s to 10s is 2.4 to 1. If you understand statistics and probability, you can understand that this is difficult to reconcile logically without some behind-the-scenes wrangling involving the Henderson (and other high-profile cards). Some of the explanations are intriguing but ultimately fail to explain the above anomaly.
As for this suggestion that PSA is hurt financially by not rewarding more 10s, it's not entirely without merit, but if they gave all those 9s 10s, they would be a laughingstock and their brand would suffer, so the mathematics used there are faulty to say the least. I couldn't really follow the rest of the argument. But let's just say you'd make a fine defense lawyer for PSA.