Who was St. Joseph's Father? The New Testament seems to give two answers

Matthew 1:16 (NASB) > **16** Jacob was the father of Joseph the husband of Mary, by whom Jesus was born, who is called the Messiah. Luke 3:23 (ESV) > **23** Jesus, when he began his ministry, was about thirty years of age, being the son (as was supposed) of Joseph, the son of Heli, The problem here is that Matthew names Jacob Jesus's Grandfather and Luke names Heli. What is going one here? A mistake? A misunderstanding of Jewish genealogies?