See  https://meta.stackoverflow.com/faq#login =>  "You must have an OpenID. If you have a registered account on any other sites in our network with at least 200 reputation, you will receive a +100 point reputation bonus when you first log in."

See also: https://meta.stackexchange.com/questions/90000/should-stack-overflow-sites-share-base-rep100-to-some-degree