She was actually confused before this impenetrability.

Remembering the awful things she had said to Her, she was suddenly conscience-stricken as she saw Ida's calm radiance of demeanor.

She began to wonder if she had not been mistaken, if Ida was not really much better than she herself.
She knew that is she had had such things said to her she could not have appeared so forgiving.

Such absolute self-love, and self-belief, was incomprehensible to her.

She had accused Ida of more than she could herself actually comprehend.