In the absence of any other answers, I'll give my guess. When you print a page from a web site, you'll probably notice that your computer accesses the web site. I think this is to get a printable version of the page. I suspect that some sites don't have a printable version of the displayed page, and so the 'Print this page' link accesses a different version optimised for printing.
This is probably total rubbish, but will at least push the thread to the top of the list where someone more knowledgeable can give the right answer.