Originally Posted by
mimik1uk:
“maybe all 1D songs just have auto-tune built in
”

. I think i actually heard what sounded like some pitch correction software being used, probably not actual "autotune" as autotune is a specific type of (extreme) pitch correction.
also you can clearly hear they dubbed the backing track over the top of her post-production so they would have had to edit her performance to keep it in tune (and in time) with the backing track.
Lastly let's not forget that X Factor producers are super talented at editing so can manipulate footage and audio as much as they like and however they like. It must be fun having that much freedom and skill, I'd actually love to be a producer in a few years time