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Same task, two run-ups. Flip between them.
The vague prompt asks for book recommendations and nothing more. Watch what plausibly follows: shapeless, hedging prose, because a vague opening continues into vague filler.
Now flip to show three examples. The task is identical, but now the run-up ends with three recommendations in a clear shape. Suddenly the most plausible continuation is a fourth, fifth, and sixth in that exact shape. Same model, same request, completely different output, and not one formatting rule was written down.