Hi Janice and welcome to MR.
The nearest explanation I can think of equates truth with integrity, specifically character integrity. If a character acts "out of character" that's a lie. OK, once, if it helps your fiction, and you make it clear that the character doesn't want to do whatever it is, but does it anyway, and as long as it's not a sort of deus ex machina. But basically, characters should act in accordance with the rules you have built up for them.
Or maybe I'm missing your point.
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