They aren’t switching though. They are given a choice between A&B then if they choose A they are given a choice between B&C. The choice between B&C should be 67/33 but instead it’s around 75/25 and the authors claim this is because in not choosing B the baby decided they liked B less. The evidence for this claim is that if an adult makes the first choice for the baby then the 2nd choice is 50/50.