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OK some of these have my personal head cannons, so here are my reason 
Carter : before he never once had a Kit-kat, so when Leila gave him one, he ate it like that, and she just gave him a glare, but he didn’t understand why, so he just continues to eat it like that and the others find it too funny.
Leila : is it the way it should be eaten. Why? Because I personally had a canon her to be more updated on trend, considered socially good  despite being an outcast herself (magggic)
Theo : same with Lelia but I headcanon him to have unhinged or gray day every blue moon and when those days happen if he decides to eat a Kit-kat that day, he’s gonna eat it like the second option soooooooooo
Ridley : she really doesn’t care in my opinion so she eats it the quickest, or most efficiently whatever that is from the top or from the side after all, she’s tired and she wants to get it over with.
Olly & Izzy : I don’t think I need to explain.