You were right the first time: she is a mix. If the same parents produce chicks with a variety of traits, then none of the chicks belong to any pure breed.
Besides, she has red earlobes and Leghorns have white earlobes. So she's missing an important breed trait.
I don't think the body shape is quite right either, but I've never been very good at recognizing correct body shapes for various breeds, so I could be wrong there.
I think people are seeing white feathers and a big floppy red comb and not looking any further. That might be enough to distinguish White Leghorns from most other pure breeds of chickens, but it is not enough to tell a Leghorn from a mix.
We do know for sure, just by looking at her and at her parents and siblings.
I think she looks much like a Leghorn, but is not actually one. For a backyard pet or egg production it doesn't matter, but for breeding it does, because she has some genes different than what a White Leghorn would have.
I'm curious: what color eggs do they lay?
Have you already found the genetics section of the form?
https://www.backyardchickens.com/forums/exhibition-genetics-breeding-to-the-sop.16188/
Of course some of the threads will be more helpful than others, but there is a lot of material over there.