These are pages listed in Google’s search results alongside normal results but labeled as a ‘Supplemental Result’. They come from a Supplemental Index maintained for lower quality pages. See the definition of a supplemental result at Google’s Help Center.

Supplemental pages are generally considered less trusted and rank lower than pages in Google’s main search index. Pages enter the supplemental index for a number of reasons:

• They have limited number of links pointing at the page (internal and/ or external).
• They are similar to other pages in a site (duplicate content).
• The URL for the page is exceptionally complex.