Why was the poet puzzled to hear the little girl's answer?

The narrator in "We Are Seven" gets into a good-natured argument with the little maid he meets. The narrator asks the eight-year-old how many brothers and sisters she has, and she tells him, "We are seven." She goes on to explain that two of her siblings have "gone to sea," and two live in another town. Two more have died and are buried in the churchyard close to where the girl lives with her mother. The narrator then repeats what the girl has told him and tries to clarify that, in that case, she has only four siblings, or five children in her family. The girl relates that she knits her stockings and eats by the graves of her dead brother and sister. She sings to them and sits with them. To her, they are still her siblings--as much as the ones who have gone to sea or live away from home.
The narrator is puzzled that she counts the dead siblings as her siblings still. He tries to explain the difference between those who live on this earth and those who are no longer present here but are in Heaven. He tells her, "You run about, my little Maid, / Your limbs they are alive." In the first stanza, a short prologue implies that the child is too sweet and innocent to understand death. But by giving the little girl the last word, "Nay, we are seven," the poet may be suggesting that the girl knows more about what it means to have brothers and sisters and be part of a family than the narrator does.