Google introduced Supplemental index in 2003 to show more accurate result for search query. Actually supplement pages are those pages that Google think important for most esoteric queries. Google also make it clear before they didn’t show supplement results simply by ignoring them. Google added supplement result in search results only to help web surfers to find more specific results and for that Google improved crawler frequency.
In July Google removed supplemental results from their index and now Google says they completed the next step to eliminate artificial difference between indices. Now they show more specific result instead of crawling some part of index now they will crawl in depth to show more specific and will not show larger set of results in search results. To improve searchers will see more results for related queries.
So from now Google comes with improved crawler efficiency that will crawl web pages that were in supplement results. It is advisable to keep eye on this new improved algorithm.