Why can't instruction books be the same as the software or the site?

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I've just added a wireless router to my computer setup and now my wife is able to use the laptop in another room while I'm here typing. The router came with an instruction book but when I tried to open up the browser page to access the site, I kept getting a message saying that it couldn't connect; yet I could access any other site on the Net easily. Turned out in the end that one of the numbers of the IP address given in the book was wrong! And then, to add to the confusion, the highly detailed explanation about how to set up the wireless configurations - with pictures - as shown in the book were nothing like what came up on the screen. Instead of screens and screens to work through - there were a couple, neither of which were anything like the book. Good grief. Do other people wonder why companies produce books that aren't the same as what you find online?