Gmail: What does it mean when you get an email from somone NEW on the same thread as Someone old?

I have Gmail and you know how gmail keeps all emails in the same thread and will bring the whole thread up to the top with a new email? I had a thread of emails going with someone whom I met at a forum. Suddenly, I get an email from someone today from a completely different email address. But it arrived in the same thread as the other emails. Why did this happen. I've only seen this happen in Gmail before.
Gmail: What does it mean when you get an email from somone NEW on the same thread as Someone old?
If the new mailer sends you a mail with the exact same subject line, it will appear in the same thread as the original emails.