Why does the girl say, ”we are seven”?

In this poem by William Wordsworth, the speaker meets "a little cottage Girl" and strikes up a conversation with her. He asks her the simple question one might routinely ask a child, "Sisters and brothers, little Maid, how many may you be?" The girl tells him "Seven are we," but goes on to say that two live in Conway, a seaport village in North Wales, and two "are gone to sea," presumably serving in the British Navy. She then easily drops the fact that two of her siblings are buried in the church-yard near her home. The speaker quickly does the math and realizes that the child has four siblings still living, which makes the answer to his question five, not seven.
He presses for an explanation. She then describes how she incorporates the dead siblings into her daily activities. Since their graves are directly outside the back door of her cottage, she often brings her sewing tasks to their gravesides and sits there singing to them as she sews. Additionally, she often brings her supper outside to eat beside their graves. The speaker tries to get the child to recalculate her original answer, reminding her that two of her siblings are in heaven. The final stanza states that "the little Maid would have her will, and said, 'Nay, we are seven!'" Clearly, she still numbers her dead siblings among her family members, and considering how frequently she includes them in her daily life, it makes sense. To think of them that way is "her will." She wants to keep them in her heart, and she deliberately does, so her arithmetic is accurate.