Why do girls bully by pretending to be sorry?

One of my co-worker's daughter is currently in 7th grade. She's been having a very hard time and there is a particular girl (along with her 3 friends whom act like silent followers; they're basically guilty of doing nothing) that's been mainly passive aggressive.

Supposedly she apologized slightly over a week ago but just yesterday, she came to her and said she was just pretending all along, that they're not her friends and had a smile on her face when telling her that. Her daughter came back home angry and hurt. She's back to being ostracized again.

What's with the plot of faking an apology for a week, then go back to bullying again. I've heard female bullies are the ones into that plot. Why pretend to be sorry, if you're going to bully again? Needless to say, my co-worker is close to either getting her daughter homeschooled or moving elsewhere. This has been going on for several months and she really thought the girl was sincere with that recently apology but it was a set-up.

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The other girl then told her they were really joking around with her.
Why do girls bully by pretending to be sorry?
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