Do you ever say someone did or said something that they really did or said, but they get mad at you for saying it/ call you a liar/ say they didn't do or say that thing you said they did or said?
Maybe because they actually forget they did or said it, maybe because they're just lying to cover their tracks?
My mother did that and still does that to me, whenever I say she did or said something she really did or said. She seems to have a very selective memory, only ever remembering the good things she does and says.