When you search for something in Google Groups, it searches in all the web forums it knows, not only in Google Groups + usenet. See e.g.:
So this is (one of) the result(s) they aimed when they added additional infos for forum threads...
(I haven't find such news either in the blog or in the forum, but of course I can be wrong. If so, please delete this post. Thanks!)
There appears to be a bug here. Try doing a Google Groups search with "Google Groups" (as opposed to "all groups") selected. Then click the "Web" link on the upper left to perform a Google Web search. Notice that all the Web search results are restricted to groups.google.com pages. Also notice that you are "trapped" in this mode for all subsequent searches you perform unless you go out of your way (by going back to Groups and selecting "all groups" or navigating afresh to www.google.com, etc.).
kevin: you're right, such a bug should have been easily avoided... still this also brings the focus on the fact that a url like groups.google.com/groups/searc ... will still state it searches all groups.
<< Also notice that you are "trapped" in this mode for all subsequent searches you perform unless you go out of your way... >>
That's because they're using the [sitesearch:] operator as a hack for this. You can simply remove [sitesearch=groups.google.com] from the query string too.