A WebmasterWorld thread is reporting many cases of people noticing the pages on their site going into the supplemental index. But even more so, they are noticing that a site:www.domain.com command returns one result and then shows you:
In order to show you the most relevant results, we have omitted some entries very similar to the 1 already displayed. If you like, you can repeat the search with the omitted results included.
And then when you click to display those results, up come pages, not necessary with the supplemental tag. Also, those pages tend to have very unique titles and content - so they are technically not all that similar.
I do have some examples validating these reports - but I cannot share them (I promised).
But then you have sites like this that do not have that problem (i.e. site:www.seroundtable.com).
Preferred member, Whitey, writes:
- Our 2nd result is not supplemental
- On some of our sites we are near 100% free of supplementals when clicking on "show more results"
- A site:tool search on a page like this site:oursite.com/Widget1/ shows the same pattern as described above.
- Meta titles and meta descriptions are unique
- Currently our traffic is increasing.
- Header text is sometimes being displayed in the SERP's "description" - Sites with good "trust rank" seem to be unaffected - Affected members reporting traffic climbed first then crashed
I have a feeling it has to do with page popularity (i.e. linkage).
Forum discussion at WebmasterWorld.