What is it with girls smirking at me?

I was standing over at a table with one of my friend and I couldn't help to notice this girl staring at me. She was sitting down with her boyfriend and she kept staring at me, and when I looked at her she kinda flinched and looked away and started talking to her boyfriend again. I often get a lot of girls smirking and smiling at me, I don't get it. I get hit on by guys a lot and I'm 100% straight By the way. but what is it with these girls and their smirk? Also, once I had this event in college and every1 came with their dress and etc, and when I came I had a woman smirking at me.
What is it with girls smirking at me?
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