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Because programmers usually think like programmers and not like users... If you want to test a program for bugs or missing features, give it to users to test. They will manage to break and find things because they do things programmers would not think about. Usually because they make no sense (at least from their perspective...).
The validator is not the final word of course. It is just checks their interpretation of the specifications. It is actually quite easy to create a books that fails on (almost) all readers/programs but validates fine.
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