Why do HPDs lie so illogically, yet so convincingly as well?

I have to give an update.
Yesterday my HPD came to town to ask me, one last time, to give our love another chance. For about 6 hours she tried to take on each of my doubts in turn, spinning this, spinning that, and all so convincingly, so emotionally (she in tears almost throughout), that although I never backed down, I did begin to feel doubt. Had I really done this woman an injustice? Sure, she had behaved strangely, what with her internet postings, etc but maybe I'd overreacted? "There's never been anyone but you! Can't you see that!" she sobbed.
I went home though, defying her pleas to stay the night.
The next day we met before she had to get on her train. She told me that a guy she had met 6 months ago was getting serious, so I needed to make a decision. Serious?
"So you've been going out?" She sighs but says nothing.
"You've already been intimate with the guy, haven't you?" She gives me this you-just-don't get-it-look as if I'm the crazy one.
Me, deciding to make it easy for her, "You've kissed the guy right? You'll admit that much, at least?"
She: "Yes."
I of course was flabbergasted - not so much by the fact that she had met a guy while we were dating, and hooked up with him as soon as I pulled out. That much, with an HPD, is to be expected. What floored me was the matter of fact way she brought up and admitted the very sort of thing that she had been denying the previous night. All night she gives me the dying-swan, lonely-me routine, the next day, I'm told to grab her before someone else does.
Me: "And when were you going to tell me about this guy? You said that you'd tell me if you got serious with anyone."
She: "I told you I would tell you if I was in a relationship."
Whereupon I up and walked out. Enough of this crazy-making crap.
But I was thinking, with HPDs, it's not just lying. It's a completely different language and moral standard that they apply to whatever THEY do.

compton wrote: It's a completely different language and moral standard that they apply to whatever THEY do.

This is classic and typical of so many interactions I've had with the HPD woman. I too have my doubts from time to time, like maybe I imagined some of it, or reading the forum is causing me to impose behaviors on her that weren't really there. Distance makes her seem less dramatic, and more humane. But reading what you wrote here just brings the truth to light. They live in a different standard of social interaction. My ex used to say we were exclusive but colliding systems. She doesn't understand my world anymore than I understand hers. In a way, reading what you wrote here as helped me to extinguish the last flicker of hope that maybe things could be different. The system of the HPD is internally coherent, there is little chance to break through it. Certainly not from the perspective of a system external to it. It's as Sofrance wrote in another post today: there's nothing wrong with you if you don't know any different.

This could lead into a very interesting philosophical discussion about whether there are objective moral principles and how we might discover them. But this isn't the place for that.

Thanks for the comments, Principled Man.
You're right, it is their world,and in it, they will always be the sole definers of what is right and wrong.

Example 1:
Mine told me that her ex had once stored some items at his former gf's house. My HPD talked about this as angrily and indignantly as if the ex had been screwing a different woman every night. But she casually admitted that this ex had "smothered" her with affection, forcing her to go out and spend the night with one of her on-off lovers. "I just needed to be held, stroked. It was nothing sexual."
Me: "So it's all right for you to spend the night with an ex, if your guy is being too 'smotheringly' attentive. But it's a crime for your guy to have a guitar in an ex's attic."
She (looking at me as if I'm nuts): "YES! There's no comparison!"
Here's the thing: there's not even a chuckle about it, as if to say, "Yeah, I know it's a bit hypocritical." There's none of that.
I should have run as soon as I heard this story.

Example 2:
They will even deny you the right to use their own language back at them, because when you use the same words, you are not understanding them in the right way.
To explain (and I swear I'm not making this up!):
She: "Yes, I still love X."
Me: "OK. Finally, you admit it. Thank you. So you still love X."
She: "No! You can't say that!"
Me: "Why not?"
She: "Because..." (exasperated look) "you don't get it. You don't understand how I love him."
Me: "Yes but you love him, right?"
She: "Stop!"
Me: "So you can say you love him but I'm not allowed to say you do?"
She: "Yes."
And this said in the most matter-of-fact way.

Principled, you have to write a diary, as I did. Because our memories are optimists. You remember the good stuff. Write down all your bad experiences in detail. Write how you were tricked again and again. And look at it whenever you miss her.