I originally posted this in another form as a comment, but I think it deserves an answer. However, I will not answer it as-asked, but as-interesting.
At least in the USA educational system, there are criteria for selecting appropriate reading materials. I refer not to intellectual, moral, or social content, but to readability.
The concept is that at any point in time, each student has a certain reading level, which can be measured. Then, the student should be assigned reading materials at a slightly higher reading level, so that the level gradually increases as more and more difficult material is assigned.
There are several quantitative methods. One of them, private copyrighted and trademarked, is known as "Lexile." There are others. The inputs to the calculation vary with the method. I believe that Lexile is proprietary, so its exact algorithm is not publicly known. Other methods are publicly known.
One possible input to the calculation, among others, is the number of different words used in the book. That may be scaled for whether the words are unusual in usage or unusually long.
However, this can be gamed. Many famous novels are simply written, despite their profound content. That was intentional. They were written in an era when education was taken seriously at each grade level, but most folks did not graduate high school. Thus, they are written so that someone at about the eighth grade level of reading (real grade 8, not "everyone passes") can read the book. But then, such books would not be sufficiently "advanced" for typical native-English readers in grade nine! Here's an example: Grapes of Wrath, by John Steinbeck.
Thus, some authors will intentionally include more words, unusual words, and longer words, in order to raise the reading level. That helps their otherwise-obscure book score better for more advanced readers, even if the intellectual content is much lower. Especially in fantastic fiction, there may be many characters with unique names (no wizard is ever named "John"), places, and even objects.