It's all about ads in Gmail
As all Gmail users can see the ads next to your email and Gmail team has decided to do something with that.
It displays relevant ads on accordance of mail say if you have a purchase offer on any products say DVD/recorder, then you could see ads relevant to that like sale discount etc.. but that cant be always and no matching ads can be found sometimes.
So in that case, It may shows you ads matched to another recent email published.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.
But they assure it follows the private policy and it's all the automated process done and no humans involved just to make sure they aren't reading your mails.
From Gmail Blog:
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