Within the announcement Google made earlier this year about the Hummingbird update is the search engine might rewrite queries, substituting some terms within them, when they think doing so might improve the results that searches see, and a very recent Google patent describes how Google might use a data driven approach to explore how effective those substitutions might be.
There is a history of Google making changes to queries and results to try to provide better search results.
Titles – In January of 2012, a Google Webmaster Central blog post told us that Google might sometimes change the title of a page in search results if they thought the new title might lead to more clicks and views of a page. While that might not be what the author of a page intended, it shows that Google is trying to make it easier for people to find the information they are searching for. I’ve run across sites where all the pages had the same titles, but unique main headings, and saw Google add the text for the main headings to those titles for each page.