Because the IP addresses in the email are Google's Web servers...because you're posting on the Web through one of Google's Web servers.
Real POP and IMAP email clients (Thunderbird, Apple Mail.app, etc.) will usually have your real IP address in the Received: headers.
However, if you email someone and link to an image from your own Web server in that email, then you can find out the recipients email address when they click to view the image or Gmail displays the image inline. Which is why I have images turned off by default in Gmail.