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It's because someone somewhere links to that file, so they treat it as content as well as its true purpose.
The best example is in the search results you posted. #1 is Wikipedia expaining robots.txt. #2 is the White House robots.txt itself.
Look again at at the Wiki article and you'll see they link to the White House robots.txt
Brett Tabke experiments with writing a weblog in a text file usually read only by robots. Trenchant commentary on the world of search engine marketing.
www.webmasterworld.com/robots.txt - 2k - Cached - Similar pages
google's robots txt - [ Translate this page ]
User-agent: * Allow: /searchhistory/ Disallow: /news?output=xhtml& Allow: /news?output=xhtml Disallow: /search Disallow: /groups Disallow: /images Disallow: ...
www.google.com/robots.txt - 3k - Cached - Similar pages