I consider the email address one of those internet secrets. Most of the time you’ll see it commenting on blogs. The commenters have something to say but want to avoid using their real email address.
I have no idea where the email address came from. I first used it on a whim without any expectation of a matching Gravatar. To my surprise who ever owns anon.com went to the bother of registering a Gravatar account. Thanks to this Gravatar icon I started noticing the address all over the internet. I can only assume other poster throughout the years have followed the same thought process to arrive at this one fake email address.