I don't think anyone knows the answer to this question - it just happens. A friend added me as an AU on his discover card for a while. We have different last names, different addresses, and live in different towns. Within 7 days, Discover was on my EX report. His address was listed under my previous addresses. He never gave them my SS#.
Sit back and watch the magic happen.
5/11/08 FICOs: EQ - 551
8/6/08 FICOs: EQ - 659, EX - 663, TU - 683