When we talk about the results that show up in search engines, we often do so in terms related to relevance and importance of those results.
Sometimes the results we see, and that we don’t see, are influenced by other factors, such as steps taken by the search engines to reduce the amount of work that they have to perform in order to return results to searchers.
Using Two Tiers of Search Results
If a search potentially returns thousands of results, and people only look at the first few pages of those results, it would make sense for a search engine to serve results in batches, and perhaps only initially use a modified (and much smaller) version of their database to answer search queries.
A first index tier may have a number of potential results pruned, so that documents that are more likely to be returned at top answers to searches are kept. The first batch of results returned to searchers may be taken from this pruned index.