Why girl says I flirted with her at the bar to a friend of mine when she did it?

The girl and I have a mutual friend. And a few days ago in a bar we were there with a group of friends, and she starts to flirt with me but she told my friend that I flirted with her. she has a boyfriend so i didn't try anything

Why girl says I flirted with her at the bar to a friend of mine when she did it?
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