As a starting point, one of Joseph's two fathers could be by simple adoption, as Augustine suggests, or more likely the special adoption by a father-in-law with no sons, or could be a maternal grandfather. On the other hand, the resemblance between "Matthan" and "Matthat" suggests they are the same man (in which case Jacob and Eli are either identical or full brothers involved in a levirate marriage), and Matthew's departure from Luke at that point can only be to follow legal line of inheritance, perhaps through a maternal grandfather. Such reasoning could further explain what has happened with Zerubbabel and Shealtiel.