While there's the possibility of an unknown open proxy on the server and/or other security vulnerability, it appears to me that the IP is forged and/or Wordpress is perhaps somehow being fooled - again I say that because that IP has a reverse record, and yet the entry you posted doesn't include that part.

Anyways, if it happens again, post here as soon as possible and I'll have the logs, etc looked into for anything unusual.

In the meantime, if your blog has an IP ban feature, feel free to ban the IPs associated with this site if you want.

Thank you for taking the time to post the details, and your patience.

Ron