Apparently it's been suggested that the island was originally supposed to be a copyright trap. According to wikipedia it's a known practice among copy protected reference works like maps, dictionaries and telephone directories to include fictitious entries; something that doesn't actually exist but is considered small and insignificant enough to not degrade the quality of the work. That way, if someone would end up copying it, the copyright holders can point at the fake entry and state that it's impossible for them to have made the same error without copying their work.
Granted, putting an entirely new island on your map would kinda be big enough thing to be considered a rather serious flaw, at least in my books, but it's possible that its origins are a result of a copyright trap that took a life of its own.
Anyways, here's the wiki article that I stole this info from. [http://en.wikipedia.org/wiki/Copyright_trap]
