It would be pretty nervy of a search engine to treat a page title as one of its ranking factors, and then turn right around and make up a different title when showing search results.
The process of discovery, indexing, and ranking and the user interface are two different things. It could make perfect sense to, say, translate a long, awkward title into plain English or to minimize duplicate page titles on a SERP by making changes (e.g., by appending site names to generic titles, as in "Fuzzy Widgets - Widget Warehouse" instead of "Fuzzy Widgets.").
When Google changes titles, it's simply doing what site owners and editors do when they link to a site with their own anchor text.