This quote from
Insight on the Scriptures p. 1189 explains it well, I think: "If there seems to be a conflict between the angel’s instructions to Mary (“you are to call his name Jesus”) and Isaiah’s prophecy (“she will certainly call his name Immanuel”), let it be remembered that Messiah was also to be called by yet other names. (
Lu 1:31; Isa 7:14) For example,
Isaiah 9:6 said concerning this one: “His name will be called Wonderful Counselor, Mighty God, Eternal Father, Prince of Peace.” Yet none of these names were given to Mary’s firstborn as personal names, neither when he was a babe nor after he took up his ministry. Rather, they were all prophetic title-names by which Messiah would be identified. Jesus lived up to the meaning of these names in every respect, and that is the sense in which they were prophetically given, to show his qualities and the good offices he would perform toward all those accepting him as Messiah. So also with his title Immanuel. He measured up to and fulfilled its meaning....With the coming of his beloved Son to earth as the promised Messianic “seed” (
Ge 3:15) and rightful heir to the throne of David, Jehovah was furnishing his greatest sign that he had not forsaken mankind or his Kingdom covenant. The title-name Immanuel, therefore, was particularly appropriate to Christ, for his presence was indeed a sign from heaven. And with this foremost representative of Jehovah among mankind, Matthew under inspiration could truly say, “With Us Is God.”