Why the sudden jump of 100 points in my reputation ranking?

(Tuesday, 30.Aug.2011, Taiyuan China time) Until a day or so ago, my reputation ranking was 774. Then, suddenly, with no activity on my part, it is 874, exactly 100 points higher. What happened?

• One way this can happen is by your logging into a second StackExchange site for the first time. Do you think it might be that? Aug 30 '11 at 1:55
• A check of your account tab: math.stackexchange.com/users/2344/mike-jones?tab=accounts says this is your only account with rep $\geq 200$; you might want to visit math.stackexchange.com/reputation to see how your rep went up. Aug 30 '11 at 2:03
• @Dylan Moreland: Yes, I signed up with Meta Stack Overflow. So, is such bumping up of rep in another site a bug, or not? Thanks. Aug 30 '11 at 3:00
• @Mike I think this is by design. See this answer on the SO meta. It happened to me when I signed up for the TeX SE. Aug 30 '11 at 3:27
• Aug 30 '11 at 10:19

This is by design. See this blog post where the feature is announced, and the official answer on meta.SO (in the "additionally" section). As both resources mention, only one of the two accounts you are associating needs to have $\geq200$ reputation.