When Google first launched the referral program for AdSense publishers, I remember exactly that they advertised "$1 for each time a user downloads Firefox through your button and runs it for the first time" and "$100 for referred publishers who sign up for AdSense and reach $100 in earnings".
It might be that my memory is short or they really changed both programs, since now "when a user you’ve referred to Firefox plus Google Toolbar runs Firefox for the first time, you’ll receive up to $1 in your account, depending on the user’s location". That's what the image above is about. I've received $0.10 for a user that downloaded and installed FireFox, probably because was not from a location "accepted" by Google. Now, I wonder on what principles are these amounts given. Do they calculate in what countries FireFox is most used and for those countries they pay less than on the countries where FireFox is not so popular? Or do they take into account the probability of receiving donations for FireFox? Who knows. Their help doesn't say much about this.
Why the "Google are smart" title? Not because Google has a smart algorithm as a search engine or because the people behind Google are smart, but because of this funny sentence I've found in their help:
I know my English is not good and that they wanted to say "What is the definition for referrals?", but that sounds just like the "Joey are stupid" line from "Friends".
Other things you might not know about AdSense, if you didn't go through the Program Policy (I plan to read their Online Standard Terms and Conditions but another time):
Labeling Ads. Publishers may not label the ads with text other than "sponsored links" or "advertisements." This includes any text directly above our ads that could be confused with, or attempt to be associated with Google ads.
Site may not include: Pop-ups, pop-unders or exit windows that interfere with site navigation, obscure Google ads, change user preferences, or are for downloads. Other types of pop-ups, pop-unders, or exit windows may be allowed, provided that they do not exceed a combined total of 5 per user session.
Your site must not contain broken links and must be launched, functioning, and easily navigable.
Sites must respond adequately to support requests and enquiries of their users.
Incentives. For example, your site cannot contain phrases such as "click here," "support us," "visit these links," or other similar language that could apply to any ad, regardless of content.
In order to avoid associations with copyright claims, website publishers may not display Google ads on web pages with MP3, Video, News Groups, and Image Results.
Code Modification. Any AdSense ad code, search box code, or referral code must be pasted directly into Web pages without modification. AdSense participants are not allowed to alter any portion of the ad code or change the layout, behavior, targeting, or delivery of ads for any reason.