Girls, Why do women use flirty body language if they have already decided a guy is just a friend?

So... I've known about body language and signs of attraction for a while now. As a man, I've been working on making my own body language signal attraction and confidence when flirting. Admittedly, when I was younger I was overly shy and nervous and it came through in my body language. Now, however I've become a lot more confident and I'm able to use open and positive body language as well as male attraction signals (that slightly longer eye contact, feet pointing towards her, leaning in, getting close to her, smiling, even the inverted triangle trick). I have also made it a conscience effort to be attentive to the body language women I'm interacting with are giving off.

Several times I've been met with lots of supposed feminine attraction body language and flirtatious signals, to only be friend-zoned (or outright rejected). It's confusing and a little disheartening. I don't claim to be a genius but I'm not stupid. Why would women actively flirt if they know you are attracted to them (which typically women pick up on attraction a lot faster than men do) only to friend-zone you when you make a confident first move? What am I doing wrong?

Girls, Why do women use flirty body language if they have already decided a guy is just a friend?
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