But I don't know enough about e-mail headers to provide an alternate explanation if there is one.
EDIT: This is that other guy's site, FWIW: http://jonathangillette.net/
And he's on twitter: http://twitter.com/jgillette
EDIT 2: The infamous Zed Shaw posted a link to another page which I'm more inclined to believe is the real _why. I'm not going to post the link here, but you can find it in Zed's twitter stream if you really want to.