Twitter is inserting tweets into user's feeds even if the user is not following them.
It's not advertising: Twitter's algorithms are selecting tweets of a users followers and inserting them into the feed.
That might explain some of the strange things i'd been seeing.
Over the last several months, Twitter has begun inserting what it believes to be relevant and popular tweets into the feeds of people who do not subscribe to the accounts that posted them. In other words, Twitter has started showing users tweets from accounts that are followed by those they follow.
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