My first guess is still the fact that all AU's and above are counted, graded and sealed and if your bill is not, it's an EF, even if it looks like an AU.
This is extremely far from the truth. In fact, professional banknote grading is too new yet to have graded even a "majority" of all AU and better notes. First of all, that is a great quantity of overall notes, and second, many notes are held in old collections and will never see the light of day, let alone be brought out for professional grading. Many collectors are quite happy with their ungraded notes. Additionally, it is incomprehensible to suggest that if a note is not professionally graded, then it is automatically no better than EF. How many ungraded Gem UNC notes would you like to see? I've got a pile of them.
Generally speaking, the prices in the Charlton guide are determined based on extensive input from members of the Pricing Panel (see the first page of the book) who monitor sales of banknotes throughout the year. Many are dealers who trade these pieces quite frequently. Auction results, including a sample of information from eBay, are also considered. Bear in mind also that the amounts listed are only estimates of value based on prior observations of supply
and demand. The prices are not necessarily based on
asking prices however. I could have a rare note for sale and ask several times the current catalogue value for it, but if there's no buyer demand at that price, and certainly no sales, the catalogue value doesn't move. (There was recently an example of that in the AUG Thiessen/Crow $2 notes). With the early 1935 notes, including many of the 1937 Osborne notes that you are so flabbergasted about, there is little supply in the higher grades, but great demand, and several recorded sales for those prices
and higher. I suggest you pay attention to the upcoming Torex auction and just watch what some of these notes sell for.
You alluded to what I will call "grading creep" whereby an EF note a few years ago is redefined as AU, and an AU is redefined as Unc-60, etc. Some people may be tempted to do this, but any skilled grader, or competent professional grading service will be able to observe an accurate grade for the note. This is quite simply overgrading, and if done intentionally is a form of fraud. That said,
caveat emptor. Learn how to properly grade a note and don't get screwed. Know which TPGs you can trust.
Do big collectors pay off Charlton to book such prices?
And next time you want to consider making statements like that, keep in mind that it could be considered slander, and that the folks you're referring to might well be reading your post, as was the case here.