I have heard that Jesus was God's first begotten Son and Archangel. Then, it seems to me to be quite easy to understand that Jesus is the 'root of Jesse'. Basically, Jesus was not the REAL descendant of David, he came down to earth and MATERIALIZED himself, or by God's power, as human being, so God just used Mary as a kind of carrier of incubator.
Angels came down to earth and materialized themselves by different ways: reincarnation is one of them. All Chinese, whether Buddhist or not, are familiar with those stories of gods and angels, who were punished by Supreme God Shang Di and driven down to earth to be reincarnated as human being, such as Pigsy, and Sandy in the romance the Journey to the West. When their mission's completed, or penalty fulfilled, they went back to Heaven.