It does not always have to be this way though. They can be restrictive, when the AR testing first hit back when I was in school, my teacher attempted to tell me I could not read 20,000 Leagues under the sea because it was a high school level reading well above where I should be. Thankfully they did not force me and just heavily tried to dissuade me I said f*** that read the book and was able to Ace that AR test.

Reading comprehension does not mean fully understanding a word, if you can read the sentence around the word you can usually figure out its meaning even if you don't know the word itself. That is what the AR tests were originally supposed to help promote and push in reading but unfortunately people kept restricting it and what level you could read more and more and more and it stopped being about the reading comprehension and just became about total understanding of all words which isn't helpful