So, just to check, (and it’s taken me about 5 read-throughs to get to this,) you mean:
When people are challenged over something, or accused of something, why do they respond with that particular phrase? You are suggesting that the phrase itself is a give away that they are in fact guilty as charged. You think that if people came up with a different phrase, it wouldn’t be as obvious?
If you are talking about on the screen though, we, the audience, need/appreciate these clues to keep up with the plot.
If you’re talking about in your real life, then it’s probably one of two things – either they can’t think of anything else to say in the heat of the moment, or perhaps, just perhaps, they are totally innocent and genuinely have no idea what you are talking about.