Just like any web page, a search engines mathematical algorithm (E.g. Google, Yahoo, Etc..) spiders the page & takes several things in account from it to go towards the overall equation (E.G. Content, Anchors, Keyword Density, Uniqueness, Etc..).
If a page is mathematically deemed duplicate, unrelated, spammy, Etc.. it will then be placed in a supplemental (Omitted) index. I will use Googles Omitted as an example here:
- ok, do a search in google for the term "Click Here"
- click the Similar pages link for the first listing "Adobe - Adobe Reader Download - All versions"
- scroll to the bottom & then go to the Very LAST index page
- Scroll to the bottom of that last page & click OMITTED pages
- You are now looking at pages that went into the Supplemental/Omitted index
Google: In order to show you the most relevant results, we have omitted some entries very similar to the (#) already displayed.
If you like, you can repeat the search with the omitted results included.
Keep in mind that backlinks may take a while to show up in a backlink tracking tool.