Unconscious and conscious are used to mediate the notion of meaning—we want meaning to being fixed and stable, so we claim to not understand what was meant. This is a convenient fiction, because it’s partially true, we generally don’t have a very strong understanding of what caused us to say something until we introspect. But it hides something deeper—we say thing to get things done, and the idea that our thoughts correspond to an assumed model in our heads. People are embarrassed when that model is shown not exist—for instance when they go back and forth on an idea it is assumed that their model is “weak” or non-existent because it contains a contradiction (or doesn’t address an observation at all). We thus assume the person is communicating with some ulterior motive. My guess would be that culture has selected for this consistency, because in order to coordinate around a narrative we need to believe that it will be passed between people under high resolution. This is obviously not always the case—cue 10,000 Chinese military stories of clever and subtle deceptions.