The Google Custom Search Engine Blog has informed that Google has added supplemental results to the results displayed in Google Custom Search Engines.
One of the common uses of Custom Search Engines is to provide site search. A number of users have suggested that we can improve Custom Search for this application by also including supplemental results in Custom Search Engine's results. We put fewer constraints on what pages go into our supplemental index and therefore there are many pages which are available only there. This is our first step to give you more results.
Google has also updated the Custom Search Engine FAQ:
Do Custom Search Engines include supplemental results?
In main web search, you may notice some results marked with "Supplemental Result" (see the Webmaster help center for more information). Currently, a Custom Search Engine will include these results only if one of the following is true:
- Your search engine includes only the patterns you specified and searches over three or fewer sites.
- Your search engine includes the whole web, giving preference to the patterns you specified.
This may be a good for the users as now it would be possible for google to show more results than they did before.