Supplemental index, which is considered to be a bit different from the main index, is acting like a demon. Why? Consider you having a large site with 100 of pages and suddenly for the site operator you seem to have 30 results coming up from the main index and rest of them being transferred to supplemental index. People say and most of them agree on the sitemap issue that it provides google a better insight to the inner pages of a site having pages deeper than two levels from the home page.
I have pointed out some more information on supplemental result based on my experience and research. The factors are noted below:
1. The problem mainly arises when you have a new domain and so the inner pages of the site do not get indexed soon. Or it may so happen that the site inner pages are too deep to be crawled.
2. Pages having similar content with that of its own site or having similar content with that of other sites are at risk to move into the supplemental index. The famous example can be domain.com
Though these are canonical issues but still this is a major issue for pages moving into the supplemental result.
3. When pages are too deep and so Google is unable to pass the PageRank value to the inner pages but remember always toolbar PR is not the original PR calculated by google for ranking pages it is just an indicative of the original PR.
4. This is the most common problem perhaps after the indexing issue when considering supplemental index. Domain name does not have the power or strength enough to pull back itself to the main index. I take the example of sites having songs. For a particular song the title and the description of the song more or less remains the same. But only that site will move ahead of the competition that will have unique description along with proper page strength.
Having said all that it happens some time that for the site operator some of the pages might me shown as supplemental result but again these pages come up for particular keyword when they are searched for in the search engine.
Really a food for thought right?
I would really invite comments on the same to gain more insight from various people. But what I feel is that if you have search engine friendly URL’s with good content proper title and meta description then you can easily avoid the supplemental issue at any point of time.