Rather than reflecting what Pearl is doing, it instead reflects what Pearl "is." The reflection shows Pearl bathed in sunlight, surrounded by flowers and foliage, like an ideal picture of innocence.
In this reflection, Hester realizes that Pearl is indeed innocent and thus separated from her mother. Though she is the product of her mother's sin, she does not and cannot share in its consequences. That is something that her parents must bear themselves. Through this realization, Hester feels estranged from Pearl. Pearl's life will go on, beyond the confines of the punishment in Boston. She will go through life untainted by the scandal, which her mother alone must bear.