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Here's my fictional idea...
Each website is worth X amount of Rank. In the old days, the formula was each page of your site earns a rank of 1. But that 'aint' so no more my little chick-a-dee.
Lets say your site is 20 pages and G says you are NOT worth 20 Page Rank points, but instead, for example, maybe G only gives you a site total of 8. So, then G will look at the traffic to each of your 20 pages and see where it goes and those pages get the divided up 8 points.
So, in this case, maybe only 5-7 pages get any rank. Say the homepage gets a "2", maybe 5 other pages get a "1" and several get a "0" and thats it. Your 8 is spent.
Which obviously leaves about 10 pages without any rank and they are given the dreaded Gray Bar.
Thats my story and Im sticking to it...
I used to think this was manual, and in the beginning it may have been. But now I think that original "seed set" has been used to generate a machine-learning algo that is intended to locate other pages of the same nature. This approach to various segmants of the algo is something that is more and more in use. All those PhD in Statistics folks need something to do, right?
For a while, my theoretical algorithm was misfiring quite a bit and the mysterious graybar disease was nearly epidemic. Just this past month, I'm seeing some of those peculiar gray bars come back to white or even green. That could indicate either that the machine-learning has improved, or something about its logic has been tweaked.
That's my story, and I'm sticking to it -- for now.
Use this search to get a list of your pages that are not gray barred.
This list will correlate well (not perfectly due to delays) with Webmaster Tools "Links", "Pages with internal links".
"Supplemental" implies gray bar. There may be more reasons for gray barred pages, and more gray barred pages, if your site is in a serious penalty condition or banned altogether.
I believe one reason a page may be "gray barred" or "Supplemental" is the keywords for the page are simply not searched for very frequently.
Of course there are the many other standard causes, duplicate content, duplicate descriptions, perhaps excessive boilerplate interlinking, etc. ....
Finally, today, one shouldn't make too aggressive a change to pages that are supplemental or gray barred, because the pages might be ranking just fine in the SERPS for their keywords. I have supplemental pages that rank well for their keyword terms and supplementals that I cannot find in the results pages without excessive effort!
we have a page that on checking just now, is ranking #8 for "widget resources" on .co.uk out of 62,800,000 pages, (fairly competitive) where widget is a 3 letter keyword, 256,000,000 pages and just about as competitive as it gets.
surely nobody would think this page is in the supplementals because it's grey barred?
Try tedster's suggestion, see if you can find your page in an AOL search? You may not be able to.
Some pertinent links:
"I think going forward, you’ll continue to see the supplemental results get even fresher, and website owners may see more traffic from their supplemental results pages."
When Google introduced the supplemental index they quickly realized some of the best "content" pages on the internet have NO PAGERANK AT ALL! With the introduction of "rel=nofollow" Google has really made pagerank meaningless. Now, everybody just hoards their pagerank (well mostly anyway).
I have seen this test stop functioning for periods of up to two weeks in the past.
Traffic never changes to them. These are unique highly linked pages that have been around for 9 years.
In my opinion whatever is causing this is more to do with looks then function and is probably geared to confuse webmasters. I mean really, out of the hundreds of direct friends I have and thousands of sub friends none have used the toolbar less webmasters/seo/sems.