Well, for one thing, are they really replying to what's being said? Or are they in fact continuing their own side of the conversation irregardless of what Mission Control was saying? I'm also sure very often the astronauts could anticipate what was going to be asked next and started giving the answer before the question was even asked.

I'd need to see an actual example of a clear-cut question-answer exchange that couldn't be explained as above before questioning this too seriously.

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...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere