Official Google Blog: The Google design, turned up a notch
During our process we focused on people’s rising expectations for search. As the web has evolved over the past decade, people have been typing more sophisticated searches and seeking out specialized search tools to match. To keep pace with rapid change online, we have teams of engineers working across Google to develop new ways to present and refine search results. Our central challenge with our latest redesign was to figure out how to squeeze all these tools and technologies into a single page.
Source: googleblog.blogspot.com