Here is a good, old, verifiable example. Valen the Outcast 1 in 2012 by Boom had a CGC only Jusko variant. The only way for a retailer to get one was to order 200 of the regular, $1 cover issue. Boom stated they printed 500 of the Jusko book, and then, along with Bleeding Cool, made a video of them burning the ungraded, unearned copies (still easy to find the video on line). Then, Bleeding Cool claimed only 25 retailers earned the book, implying that the other 475 were burned. But a month or two later, the CGC Census showed 75, which would make you think that even though only 25 were earned by retailers, Boom had to have more available as they may not have known how many they would need ahead of time.
Now, jump ahead to 2019. 25 earned, 75 in the census, most of us would think there were 75 max in the world. Boom decides to sell it's back stock of retailer incentive books and offers 152 Valen the Outcast 1 CGC only Jusko variant to Jesse James Comics, which he purchased and is currently selling them on ebay. I got a few of them, and found that none of the CGC certification numbers are in the Census, so they have to be added back in to prove they exist!
So, what I have learned from this is trust nothing you read, nothing you are told, and nothing in the Census. Clearly, at some point, CGC either removed 150 some books from the census, for some unknown reason, or they never added them, again for some undisclosed reason. I have asked and gotten no real response. I believe Boom sent in 500 Valen 1's Jusko variants, and had them prescreened. Then, they burned on video the books that were not 9.8 candidates, slabbing the rest, for a total around 225 9.8's.
I don't think anyone was trying to pull a fast one or cheat anyone. After all, it was Valen the Outcast, not a Spider-Man book. And I think someone at both Boom and CGC made some mistakes when it comes to the census, and just didn't care enough to fix it.
But, when you think you have on of only 10 or 50 or 100 or even 500, don't you believe it!