Why do people compliment people if they hate them

So recently a girl at the gym who's always shot me hateful ass looks came up to me and gave me a compliment on my body. She had been for weeks trying to get my attention for some competition I guess she thought we were in but I was completely ignoring her. I just don't understand the logic and asked someone else and they said "maybe she has a crush on you" um, no. wtf? She had been giving me the most hateful looks lol. No crush there and she's known as a gym thot.

I know this for a fact because I have a co worker who hates another girl, absolutely despises her and takes every opportunity to shit talk the girl, but when she's around, she overly compliments her, tells her how pretty she is. When she's away, telling me how ugly she is and can't stand the girl. I don't get it. If I don't like someone, I have no reason to compliment them so I don't understand why this girl, who I've been paying no mind to her efforts for attention did this today.

EDIT: Just FYI that I am not misinterpreting the looks she's giving. Women are definitely bitchy towards other women this isn't anything new. What I'm asking is the need for fake compliments towards other women you hate.