It's random. It happens. I get spam to my email address which isn't even my email addrress, if that makes any sense. As much as i'd love to blame Renderosity for some security leak, it's more likely the spam has some other way of finding you.
For every spam email that hits a target, there are probably thousands which miss. Random character generators and all that.
The best defense is to never reply to any of it, lest your email become classified as active.