Take a look at this link where Google datacenter IP addrresses are listed in detail

Google has quite a few seperate datacenters (DCs), each of which contain the entire index and the entire algorithms. To all intents and purposes, they are independant of each other. They don't all contain indentical indexes, and they don't all contain identical algorithms (programmes that do the rankings). It means that they often produce different results to each other.

When you do a search, you get the results from whatever datacenter Google chooses at that time. Unless you search a specific DC's IP address, Google chooses the DC to return the results from, and they choose it with every search you make, including when you click to get the next page of results. It's not uncommon for the next page of results to be provided by a different DC than the previous page of results.

Here are the IP addresses for the google datacenters