Assessment of reading is not so much the length of the book being read, but the comprehension of the text, the fluency and expression of reading out loud etc.
I could give the following text to DS2 (age 7) to read and I know he can 'read' every word. It might even sound like he understood it, until I ask him questions on it...
"Textpresso is a powerful tool for the neuroscientist due to its ability to query the full texts of tens of thousands of articles and abstracts as well as its capacity to include semantic concepts in searches. We are planning to expand the corpus to several hundred thousand full text research papers and are currently researching how scaling the corpus to this size will affect the performance of the system. In addition, the large corpus can be subdivided according to research themes, and the sub-corpora should be made available separately for searching to gain even more specificity. We have previously developed document classification algorithms (Chen et al. 2006) that can easily be applied to this task. Finally, we would like to explore the opportunity to interact with the NIF interface via NIF concepts. NIF currently queries Textpresso via a set of terms concatenated with Boolean OR or AND, which becomes unfeasible when several dozen terms are included. Concept-based queries are much more efficient and a natural way of querying. The NIF interface would then query Textpresso by passing a NIF concept ID. This ID is mapped to a corresponding Textpresso category whose lexicon has been filled with NIF vocabularies beforehand. A NIF concept search then simply becomes a one-category search for Textpresso."
I imagine that everybody reading the above text on MN will have different level of understanding of the above text. Some will understand it implicitly and be able to describe the concepts behind it in detail, some might be able to guess what they are from the context, others might just give a
face.
In the same way a child at school can be given the same set text to read, and answer comprehension questions at a completely different level.
That is not to say that a child isn't reading the 'wrong' level books at school, but it does depend what they are reading the books for. e.g. are they working on reading out loud with expression, or on inferring meaning from text etc.