The Google maintained a supplemental index that to keep search results that dont quite have enough juice to make the main Google index. But according to a Webmaster central post, Supplemental goes mainstream, the gap between supplemental and the main index has been narrow and does not show as supplemental in your search results. But it is there, and if you are bored or extremely intelligent, you can guess where the results for that search came from.
To get more information such as the supplemental index (read more - and here's Matt's talk about it on video) and countless comments could be found by following the link below.
Happy Google feeding
Official Google Webmaster Central Blog: Supplemental goes mainstream