Ever wonder why when you search for something in Google with and then without quotation marks, the number of results increase.
Specifically, when you search for a keyword phrase without quotation marks the number of matches in Google may sometimes be less than when you search with quotation marks?
The answer is because Google will often search deeper into their index to find more relevant matches when you use quotation marks in your search. When you do not use quotation marks, Google doesn't always search into deeper index tiers.
Googler Kousha explained it in a very clear and easy manner in a Google Web Search Help thread. He said:
In a few cases, this actually may happen because of the way we fetch results for you. With the high volume of sites available to search, Google separates its index into tiers so that more relevant documents can be refreshed at a higher rate. If you use quotations, we will search through more of the tiers to find as many results that fit your specific search. That means that it's possible for a search with quotations to dig deeper than a similar search without, and potentially return a higher number of results because of it.
Now, I don't think we still have a "supplemental index" but clearly Google has different tiers of indexes that they update more or less often and search more or less often based on query type.
This is common knowledge to many SEOs but it is always good to offer a refresher here.
Forum discussion at Google Web Search Help.
Image credit to BigStockPhoto for database tiers