[It] was ascribing to her a power of thought~

The protagonist, Philip, came to Ferne with his middle aged friend, Athenly, and his family to pick hops and take fresh air, where the Athenlys goes every autumn.
Philip had intercourse with Athenly's eldest daughter Sally in spite of himself on the second night at Ferne.
The next morning, Philip, Athenly's children and Sally is going for a swim to the sea.

He had expected the incident of the night before to have caused some revolution in her, but it was just as though nothing had happened; it might have been a dream; and as he walked along, a little girl holding on to one hand and a little boy to the other, while he chatted as unconcernedly as he could, he sought for an explanation. He wondered whether Sally meant the affair to be forgotten. Perhaps her senses had run away with her just as his had, and, treating what had occurred as an accident due to unusual circumstances, it might be that she had decided to put the matter out of her mind. It was ascribing to her a power of thought and a mature wisdom which fitted neither with her age nor with her character.
[Of Human Bondage by Somerset Maugham]
I'd like to know what "it" refers to.
Thank you in advance for your help.