John Lightfoot on Luke 2:4

[Because he was of the house and lineage of David.] We read in the evangelists of two families, that were of the stock and line of David; and the Talmudic authors mention a third. The family of Jacob the father of Joseph, the family of Eli the father of Mary, and the family of Hillel the president of the Sanhedrim, "who was of the seed of David, of Shephatiah the son of Abital."

I do not say that all these met at this time in Bethlehem: [It is indeed remarked of Joseph, that he was "of the house of David"; partly because he was to be reputed, though he was not the real father of Christ; and partly also, that the occasion might be related that brought Mary to Bethlehem, where the Messiah was to be born.] But it may be considered whether Cyrenius, being now to take an estimate of the people, might not, on purpose and out of policy, summon together all that were of David's stock, from whence he might have heard the Jews' Messiah was to spring, to judge whether some danger might not arise form thence.