I know, and I'm also at 4 other forums. But I thought you might have meant that we're getting posts from the sister sites. Here's why:
I've been working with a poster here, and every time I get an email notice, it links me to toms in UK. Our thread is there too, and when I reply to her, I'm told to login, then that I'm not a member (I have permanent login here in my cookie.) I'm confused. It's not a similar thread where she posted the same Q, it's the same one. I have to close it, go back to this site, and find the same thread to reply in.
The UK and US forums use the same databases although the UK forum has one extra category full of spam. The actual website software is different, and the UK one is unusable IMO. You get UK forum notices if the reply was posted from the UK forum, and since they are different domains there are separate cookies maintained for each. Therefore if you are logged into one you are not necessarily logged into the other.