The answer is quite simple: in a tutorial, or in a book covering technology X, there is necessarily more material, than what is found in the reference documenta

The answer is quite simple: in a tutorial, or in a book covering technology X, there is necessarily more material, than what is found in the reference documentations and specifications of said technology X. That's because technical documentations are concise (if only to avoid inconsistencies). Unfortunately, most technologies start with references that are already over 1000 pages! Check any standard or language reference, or framework reference. There are several frameworks with more than 1000 classes! imagine how many printed pages for the reference documatation! A book about those frameworks that wouldn't even be exhaustive could very well be over 10,000 or 100,000 pages.