Edit: Actually, I think your premise is wrong. Children don't read words they don't understand "like muscle memory". The differences in meaning between, idk, betrothed, engaged, wedded and living in sin are superficial but they carry with them implications of social value, history and more. Children are supposed to read books and pick up context and understand meaning as they go along. We don't expect a 10 year old to grok calculus any more than we expect them to grok social/sexual arrangements. But we have different standards for 19 year olds - in the case of sexual arrangements it's obvious why. Perhaps less so for calculus.
Reading and comprehension levels are hard to guage for this reason - at a certain point the words are placeholders for political positions. And it's hard to tell if someone does not understand the word or disagrees with the politics. Please see every election for past ten years for reference
But there is a reason 2nd graders dont do book reviews of classic literature. Even though they can absolutely read the complex words out loud on autopilot any day of the week.