From time to time, we run live experiments on Google — tests visible to a relatively few people -- to discover better ways to search. We do this because there’s no good substitute for understanding how real people, in real-world situations, actually operate. Theories are fine, but "improving the user experience" really happens best when we understand what people do online.
So to learn more, we sometimes randomly select a group of people to see a possible improvement to search options. Or we may select a group of people and try out a new element while they're searching. If you ever wonder why your Google site looks slightly different from that of the person sitting next to you, this is why.
This is the first time Google stops to explain users their interface tests.
More details:
Extended snippets
Try new Google design
Other version of the design changes
