Max, here's a partial screenshot of what I see when I log onto Google Groups through Firefox:
I believe, if I'm not mistaken, do correct me if I'm wrong, that you are not actually logging onto Google Groups. You are in the GMail interface. It offers a different interface to Groups. Although I have a GMail account set up (obviously, you can see that my E-mail address is at GMail), I never use the GMail interface to post here. The private message I got from you recently did come to me alone. The message headers showed only me as the addressee. However, I believe when you click the "other" button, whichever one that is, for posting in the Group, it seems like that not only posts in the thread but it also sends a duplicate E-mail to me in private. That's what happened with your most recent post here, & that seems to be the way other posts from you have been. I am generally not subscribed to threads I post in. I usually E-mail my posts to a thread from my E-mail client app on my desktop system. I don't log on here to post . . . usually. I am logged on at the moment to post this, but that's just so I could capture that screenshot. Rather, I let Google send me a summary E-mail once a day instead of having it send me an E-mail with every post that appears in the Group.
So I think the interface you are using works as you always believed it did. It's just that you are not using the standard web interface to Groups but rather the GMail interface to Groups. They are apparently different. Am I close to right about this?