As you are no doubt aware Google is a large organization and maintains servers in many countries. Even within a specific location there are likely to be several IP addresses that are used to spread the load. Which one you will get depends on a variety of conditions including which country you are in, the URL you specify, which DNS servers were involved, and how they are configured. This is common when dealing with large organizations. The details are complex and I will not attempt to describe them.
I pinged google.com and google.ca, with and without the www, each had a different IP and all different from yours.
Note that many Internet hosts, including most Microsoft websites, do not support ping. There are security implications and this is just a part of keeping a very busy server up and running.
Edit: To understand all of this you first have to understand how the DNS system works and that is far too complex to describe in a forum post. DNS is what translates a name like google.com into an IP address that is used on the Internet. The Internet could function without the names at all, the problem being that people more easily remember names than long numbers that could have as many as 12 digits.
