A Google search can be made with different search operators. The quotation mark-operator: [ “keyword” ] can be used to search or filter for a specific word or sentence. In this case we talk about an “exact match.”
When you put a word or phrase in quotes, the results will only include pages with the same words in the same order as the ones inside the quotes. Only use this if you’re looking for an exact word or phrase, otherwise you’ll exclude many helpful results by mistake. – Google Online Help
When you use the exact match search operator, the number of search results usually become smaller as you are filtering and restricting the results in the first place.
Sometimes, however, it may happen that a Google search with the quotation mark operator [ “keyword” ] delivers more results than the same search without it [ keyword ].
In our example, a Google search with the quotation mark operator (exact match) for [ “opel astra” ] returns more results than the same search request without the quotation marks [opel astra].
Exact match searchqueries will sometimes dig “deeper” into the index
Google explains this phenomenon by stating that exact match search requests sometimes make them look deeper in their index. This is done to ensure that the searcher is shown as many search results that meet his search criteria as possible.
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.– Kousha Navidar, Community Manager, Google Search
Google separates its massive index into several layers, which are also known as TIERs. Because of the multiple levels of the index, Google can update more relevant documents at higher speeds (Cue: Google Caffeine Update).
When you use the quotation mark operator within a search it can happen that Google searches through multiple levels of their index in order to deliver the best “exact” search results. This deeper level search through the index may occasionally find more qualified results than with a “regular” search of the index, without the exact match search request.