Yesterday I stumbled over an oddity that I mentioned in another thread. Using a site: query, the url in question was marked as a Supplemental Result. Using a regular keyword query, the exact same url was returned without the Suppplemental Result tag.
I know that this url is getting Google traffic, and that traffic is also converting. That is why it freaked me out, at first, to see it labelled Supplemental in a site: query. Except I now see it isn't always labelled as Supplemental and I am getting a bit confused. Is what I saw a bug? A feature? A temporary disconnect between two parts of the Google backend?
On reading your post, I rushed to my machine and searched for my site listings and I've only got 11 more supplementals to go before I can feel clean again. At my peak I have been as high as 176 supps, so a real big change.
p.s. It doesn't really bother me though, I don't get any hits from Google anymore ;-) But thanks for the tip off anyway.