I am not excusing her behaviour (and it would have upset me too), but some people do seem to operate on a different plane to others.
I have a friend who is the same - we used to have lunch every week, and she was always late with some excuse. I remember waiting for 15 mins in the rain, and she sauntered up to say that she'd seen a dress she like in a shop window and stopped to try it on! When we were out, if she saw someone she knew, she would leave me standing whilst she chatted to them, not introducing us or including me in the conversation - just rude.
At the same time, she would do very kind things, and was often very caring. It was as though she didn't realise that what she was doing was annoying and hurtful. I don't know if it was that she hadn't been brought up with manners, or quite what the explanation was, but in the end I stopped the lunches. I'm not one to cut out friends as a rule, but it reached the point where I was annoyed more than I was pleased to see her.