I figured you would have got this from the previous thread. They don't. Most people (especially women) don't even realize they are doing it. When they say "Sorry, something came up, I can't do anything with you" they believe they are being perfectly forthcoming and that anyone should be able to figure out what they meant by that.
It's the same way that I got yelled at by my mom and my ex-gf for not doing things I wasn't even asked to:
Mom: "You didn't clean your room."
Me: "You didn't asked me too, and it's been like this for the last 2 months, I didn't even think it needed cleaning."
Mom: "I shouldn't HAVE to ask, you should know."
Ex-gf: "I've been home alone for the last 4 hours, crying."
Me: "Why would you do that? I asked if you wanted to do anything tonight or if I could go play Magic the Gathering with my friends for a couple of hours. You said that you didn't care, and I could go if I wanted to."
Ex-gf: "Yes, and you should have known I wanted you to stay here with me."
They get angry or frustrated because they think differently than we do. They believe we are betraying them by purposefully not doing what they asked us to, even if they didn't use any words to ask.
The ones who DO know that they are sending mixed signals. Rarely this happens. Even when it does, it is something who believes that interpersonal relationships are a game of sorts, say the right thing, do the right thing, then it gets you what you want. Min-Max it correctly and your life and relationships are perfect. So, if you lie to someone to get what you want and they don't know its a lie, then you win. They don't feel guilty because they got what they want and they feel that the other person was doing the same to them anyways, so they are even. Everyone lies, so they have no reason to feel guilty (or so their way of thinking goes). If the other person is too stupid to figure it out, that's their fault and they need to seek therapy for not understanding them well enough.