The same plot, told three different ways
A story for a four year old is not a story for an eleven year old told shorter. It is written differently from the first sentence.
One setting, chosen when a book starts, that changes how every chapter in it is written.
The three bands
Ages 3-5
150 to 250 words a chapter. One idea a sentence. Familiar words, repeated phrases to join in with, and nothing frightening left unresolved by the end of the chapter.
Ages 6-8
250 to 400 words a chapter. Full scenes with dialogue. New words used in a way that explains them, jokes a six year old will get, and a small problem solved each chapter.
Ages 9-12
400 to 600 words a chapter. Description that does work rather than filling space, characters who want different things, and a chapter that ends somewhere the next one has to pick up.
Why it is fixed per story
Because a book whose chapters change reading level halfway through is a book that stops working. The band is set when you start a story and stays with it, so chapter twelve reads like chapter one.
If a child has moved up, start the next book in the next band. Nothing stops you having one story going in each.
Pick for the child, not the age
The bands are named after ages because that is what people search for, but they are really reading levels. A nine year old who finds reading hard has a much better evening in the 6-8 band, and nothing on the page announces which one you picked.