Google Webmaster Team has blogged about something we have known for a long time now.
Google changes page titles on search result pages sometimes to reflect the content of the web page.
Google said that they end up doing it around half of the times! The company said that this helps in offering a better experience to the web users searching for something specific.
Google developers said:
We use many signals to decide which title to show to users, primarily the title tag if the webmaster specified one. But for some pages, a single title might not be the best one to show for all queries, and so we have algorithms that generate alternative titles to make it easier for our users to recognize relevant pages. Our testing has shown that these alternative titles are generally more relevant to the query and can substantially improve the clickthrough rate to the result, helping both our searchers and webmasters. About half of the time, this is the reason we show an alternative title.