My crush friendzoned me. However when other guys hit on her, why does she suddenly start flirting with me in-front of them?

My long time crush friendzoned me, we are still good friends. However I have noticed her inconsistent behavior of suddenly flirting with me at times and other times just acting cold.
I finally realized that she flirted with me openly when other guys are hitting on her (she is a pretty girl and gets hit on by guys often) and when I flirt back I would usually get a cold response from her.
After friendzoning me this is not a cool thing to do. It hurts my feelings. Why does she do this?
My crush friendzoned me. However when other guys hit on her, why does she suddenly start flirting with me in-front of them?
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