rrbrigi I think the thing is that reading schemes and book banding methodologies each vary in the methodologies used to determine which books are easier or more difficult for someone to read.
There are a lot of factors that go in to this, length or words, ease of decoding, whether the word is common, sentence structure (short sentences are easier), length of text on pages, length of story, length of chapters, typeface, size of print, maturity of themes, complexity of imagery.
Some reading schemes also match their own structured methodology for teaching reading to the scheme, so early books will include words that their early lesson plans would include, or include material for comprehension questions covered at a particular point in the curriculum.
In my opinion problems can occur when teachers adhere to the banding methodology or reading scheme too strictly.
Each child experiences different language variations, different words may be common for that child (regional variations, out of date language in old books etc).
A child may have grasped phonic and comprehension skills for reading but their eyesight may be less well developed, so they struggle with small text and large paragraphs.
Or their concentration span might be less well developed to maintain interest for longer stories.
They could have very good reading skills but cannot understand more advanced books because the themes are too mature and they have not had enough life experience.
There are so many factors when deciding how 'difficult' a book is compared to another it is no small task to standardise the levels for each factor to produce 'stages'.