She doesn't know what she thought the first time she'd seen Becky. She knew that some people were worse off than others, and had servant before, even back in India. The difference was that in India there were so many people worse off that no one treated them any differently.
Sara never had.
So she didn't quite understand what she was seeing, this utter distain for a girl whose fortune had not been her own and had been chosen for her. It could just as well be Sara herself. What was the point of mistreating her for it?
She couldn't understand why no one else could see that.