they were just as likely to pick the wrong ones as the right ones.

[keannu]

If the noises affected their choosing specific ones after waking up, why did they pick the wrong ones for guessing the sounds they heard during sleep? The results seem quite contradictory to each other, maybe this is not an English problem, but a psychological phenomenon, but I need native speakers' instinctive opinion.

ex) Researchers at Northwestern University taught 12 subjects to associate 50 images with specific positions on a computer screen. When the subjects saw each image, they also heard a matching noise―for instance, on seeing a cat, they heard a meow. Then the subjects each took a 60-to-80-minute nap. While they were in slow-wave sleep(a deep-sleep phase marked by slow electrical oscillations in the brain), the researchers played the noises that matched 25 of the images they had been studying. On waking, the subjects were asked to perform the same image-matching task. They were much better at correctly placing the images for which they had heard the noise cues while they napped. The participants reported they had no idea sounds had been played during their naps, and when (asked) to guess which sound cues they heard, they were just as likely to pick the wrong ones as the right ones.

[SoothingDave]

If the noises affected their choosing specific ones after waking up, why did they pick the wrong ones for guessing the sounds they heard during sleep?

Because the sounds played while they were sleeping did not affect their conscious mind. They were not aware of the sounds played while they were sleeping, hence they did no better than random at guessing which ones were played.

[keannu]

Because the sounds played while they were sleeping did not affect their conscious mind. They were not aware of the sounds played while they were sleeping, hence they did no better than random at guessing which ones were played.

But it's saying the noises affected their good guessing.
On waking, the subjects were asked to perform the same image-matching task. They were much better at correctly placing the images for which they had heard the noise cues while they napped.

[SoothingDave]

Yes. It affected something in their minds that was not in their conscious mind, part of their brain that they were not aware of.