Sometimes, there aren't any good ads to match to a particular message. From now on, you'll sometimes see ads matched to another recent email instead.It is a significant move toward personalized advertising and, as the Google post notes, is a big change for Google, as they previously "had specified that ads alongside an email were related only to the text of the current message." For example, here, Google says, "Ads and links to related pages only appear alongside the message that they are targeted to, and are only shown when the Google Mail user ... is viewing that particular message."
For example, let's say you're looking at a message from a friend wishing you a happy birthday. If there aren't any good ads for birthdays, you might see the Chicago flight ads related to your last email instead.
For more on personalized ads that target not only the current content, but also to previously viewed content that has strong purchase intent, please see my July 2007 post, "What to advertise when there is no commercial intent?"