It is the latest update being rolled out to different google data centers. From Matt Cutts Blog...Big Daddy is a "new infrastructure and is not just a data push or algorithm update"
"main google.com" draws from many datacenters. some of these datacenters have changed over to using the data, algo, and structure etc. of Big Daddy. So results will vary depending on what datacenter your search accesses.
Main google is just a source that collects all the data from various data centers. All the data centers are updated when an update is on. The newest update is named Big Daddy. It was supposed to have immense changes. Hope it didnt proved bad to the ethical seo'ers rank...