In addition, Google has updated the FAQs to explain that the supplemental results are shown only if one of two conditions are met:
- 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.
I have confirmed spotting supplemental results in some of my Google CSEs. Google says that this is good because now it can show you "more results" than they did before.
Google, Google Custom Search Engine, CSE, Search Engine