Supplemental results are filtered by search engines in order not to spam users with similar pages that exist on one site or on several sites. that is why even though supplemental results are retrieved together with the main results, they are viewable only if you choose to look at them. (or) filtered results that are similar to the ones already shown.
Supplemental results usually only show up in the search index after the normal results. They are a way for Google to extend their search database while also preventing questionable pages from getting massive exposure.