I don't think any of us said just having the right attitude is enough to pass as it is not. Having the right attitude just makes it easier to pass. You walk around thinking you are a guy, despite the girlie clothes you will behave like a guy. Instead walking around thinking that you are a girl you will also behave more girlie. We all aim to "pass"; we all also had times where it just wasn't possible.
When you look around when you are walking one can see rather masculine looking girls that really are girls. Does anyone annoy them, no.
In the end, what is passing? What does it really mean? A genetic male will never be able to pass as a girl 100% in all situation, it is just not possible, because of the chromosomes his carrying. Having a an F in your ID card while having a human created vagina, is that passing? I don't think that is the point of passing at all. Passing should be nothing else that you can enjoy what you are the way you are dressed without people annoying you or giving nasty comments at you. All those people you pass outside, most of them you'll never even really see again, so what is the point of actually caring if they really think you are a girl as long as they live you alone and treat you nicely.
Because of this I think the attitude you have is a big part of if you can pass or not. If you can't behave naturally in femme clothes, you cannot really pass ever no matter how girlie you look like. The body language is a big part of our lives, we all have learned how to read it, even though no one has thought it to us. When someones unsure or nervous or whatever it can be often read by many other people. That leads to complications and nasty comments as the other people think they are better than you are etc. So to pass as a girl one has to look "passable" with that I mean there's not five o'clock shadow, using feminine cloths etc. also the body language has to fit the sex you are representing as the body language is different for males and females, if you haven't noticed.
What I'm trying to say here is that even if you are a genetic girl and you would totally behave like a male someone would suspect her being a crossdresser, even though she isn't one.