Symmetry. The two series-derived hexachords of the passage are related by inversion around the C♯ and E they share. That underlying inversion is at least partially suggested by the registral arrangement of the notes: the highest two notes of the harp melody are inverted onto the lowest two notes of the chord; the middle three notes of the harp melody (forming a diminished triad) become the middle three notes of the chord; the lowest harp note maps onto the highest note of the chord.