According to an investigation by Wall Street Journal, Google and at least three other smaller web ad networks (Vibrant Media, Media Innovation Group and Gannett PointRoll), have purposely overridden Safari's browser privacy settings using code that misrepresents its ads as being a user-initiated form submission.
The default settings of Safari block cookies "from third parties and advertisers," a setting that is supposed to only allow sites that the user is directly interacting with to save a cookie (client side data that remote web servers can later access in subsequent visits).
Advertisers like Google save cookies on users' browsers so they can track their browsing habits across the various websites they place their ads on, and these "third party" cookies are expressly what the setting is designed to block.
The report notes that "Google added coding to some of its ads that made Safari think that a person was submitting an invisible form to Google. Safari would then let Google install a cookie on the phone or computer."