Why Does Google Want To Know How You Heard About The Zeitgeist?

I am not sure I understand this one. Jessica from Google posted in the Google Web Search Help forums asking forum members how they heard about the Google Zeitgeist.

She wrote:

With 2012 coming to a close, Google released its 12th annual Zeitgeist a couple of weeks ago. We're curious to know how you heard about it and you can tell us here. :)

You can see the form over here but I don't get why Google is asking a subset of users this question. Google has referrer data, heck they know more about where you've been on the web than you know yourself.

The form asks:

We're interested to hear about where you heard about Google's 2012 Zeitgeist (www.google.com/zeitgeist/). Please tell us below!

Family member or friend?

Your social network, like Facebook, Google+, Twitter, or Reddit? A blog?

An online article from a source like NYTimes, Slate, Wired, etc.? Somewhere else?

Why do you think Google posted a form asking about this?

Forum discussion at Google Web Search Help.