Your cynical explanation doesn't fit with the actual behavior. They sort it so things they think you'll like most will be at the top, but once you've seen every

Your cynical explanation doesn't fit with the actual behavior. They sort it so things they think you'll like most will be at the top, but once you've seen everything, they don't change the order it again. If you always scroll down to where you left off, you will always see everything, and there won't be older content mixed in with the stuff you're seeing now. Given a set of ordered posts you might see ACDB, then when you check in next you'll see HFGEACDB whereas in a chronological timeline it'd end up at HGFEDCBA, but once you've seen that out of order "DCBA>ACDB" section it stays in that order.