A Google paper provides more information about Google's experiments. Google tests many new features on a subset of users and that's the reason why you may see a different Google search interface, a new background color for Google ads or more Google search results.
"At Google, experimentation is practically a mantra; we evaluate almost every change that potentially affects what our users experience. Such changes include not only obvious user-visible changes such as modifications to a user interface, but also more subtle changes such as different machine learning algorithms that might affect ranking or content selection.(...) An experiment in web search diverts some subset of the incoming queries to an alternate processing path and potentially changes what is served to the user. (...)"
More Info: Google paper (PDF)