Google has made some changes to the way ads shown to Gmail users are selected. If you use the service, you have probably noticed that Google often shows ads that are somehow related to the content of whatever piece of email you are currently reading. If you have noticed this, you have probably also noticed that sometimes the ads don't seem to have anything to do with your conversation whatsoever.
Google has decided that instead of showing completely irrelevant ads in such cases, it will just dig back into another conversation from your Gmail usage, and show you ads related to that. This is illustrated in the clip below.
"Let's say you're looking at a confirmation email from a hotel in Chicago. Next to your email, you might see ads about flights to Chicago," explains Gmail Product Manager Steve Crossan. "But 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. 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."
Google is careful to point out that it doesn't store any extra information to show these ads. It just picks a different recent email to match. It's all automated, and no humans are involved in picking the ads. The process is compared to Gmail's spellchecker. Google also notes that no email or personal information is shared with advertisers.
The change will be rolling out over the next several days, so if you are still seeing ads completely irrelevant to any conversation you've had, you may not have the new system implemented yet.