Why would a girl give extremely obvious signs that she likes me but then tells me she's interested in someone else?

This girl I've never talked to much started liking me: Her friend (s) would laugh around me, she would constantly stare at me and sometimes smile at me and blush and walk off, asks some of my friends where I am when I'm not available, checks me out by looking up and down whilst looking embarrassed.

I have tried to talk to her sometimes but she would be looking down her phone and stay rather quiet then walk off. I would also say hi in the corridors and she would look noticeably nervous (her friend laughing by her side). One day I asked whether she could sign my shirt because it was a celebration day. She looked obviously nervous when she saw me and said she was in a hurry when it was obvious she wasn't at all in a hurry. All she said was that, "She'll sign my shirt later." Which doesn't make sense because that's the only day you'd be able to sign my shirt, but she never signed it but I did sign hers.

I messaged her via text, because she obviously wasn't very comfortable talking to me, after talking for a while via text (which she did seem more comfortable with) I asked her she was interested in me, which she replied with, "Sorry I don't see you in that way but I see another boy in this way. I was just trying to be friendly and sorry that, that misled you." I then said backed off a bit and she hearted my last message. Even after saying that, the next day she creepily still stares at me even through the windows.
Why would a girl give extremely obvious signs that she likes me but then tells me she's interested in someone else?
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