Unlike the other groups in the portal, the Annunciation does not adapt the common medieval and renaissance iconography of this subject. Traditionally, architectural or other elements established a prim vertical divide between the space occupied by the Virgin and that of the angel Gabriel,  the hand was raised no higher than necessary to signify the greeting "Ave Maria," and the fingers were arranged either with index and middle finger joined to signify the two natures of Christ (viz. the 13th-century glass in the Cloisters) or with the index alone pointing to Heaven (as in Memling's painting in the Metropolitan Museum). Nor was it common for the Virgin to bow so deeply (but see Botticelli's Annunciation also in the Metropolitan). And, of course, Gabriel had wings. 

The idea may be to introduce a gesture of humility before the great dignity to which human nature is raised by the Incarnation, a dignity which is celebrated in the Nativity and Coronation of the Virgin groups which are respectively below and above this one. The gesture of bowing at the thought of the Incarnation is repeated still in the Catholic mass when the faithful recite the Creed's words, "by the power of the Holy Spirit, He was born of the Virgin Mary."

More of the Annunciation