That is a very good question! I think we will see that Mary did indeed believe what the angel had told her but could not understand how it would happen. We will now look at both passages.

Let’s read Luke 1:13, 18-20, “Do not be afraid, Zacharias, for your prayer is heard; and your wife Elizabeth will bear you a son, and you shall call his name John…And Zacharias said to the angel, ‘How shall I know this? For I am an old man, and my wife is well advanced in years.’ And the angel answered and said to him, ‘I am Gabriel, who stands in the presence of God, and was sent to speak to you and bring you these glad tidings. But behold, you will be mute and not able to speak until the day these things take place, BECAUSE YOU DID NOT BELIEVE MY WORDS which will be fulfilled in their own time.” It is clear that Zacharias DID NOT BELIEVE God’s word. Some have said “he DOUBTED God’s Word,” but the angel dismisses that thought by charging Zacharias with UNBELIEF. If Zacharias was familiar with the appearances of Gabriel in Old Testament times (see Daniel 8:16; 9:21), he should have known that Gabriel was indeed God’s “Messenger angel to the nation of Israel” who was sent on various occasions to give them “God’s Word.”   Perhaps we can relate to his lack of faith, for we too are prone to focus on our “human weakness” instead of “God’s power.” But that is no excuse, for when God speaks in clear and simple words, WE OUGHT TO BELIEVE HIM! Zacharias should have believed God and he paid a price for not taking God at His Word.

Now let’s read Luke 1:30-31, 34-35, “Then the angel said to her, ‘Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bring forth a Son, and shall call His name JESUS’…Then Mary said to the angel, ‘How can this be, since I do not know a man?’ And the angel answered and said to her, ‘The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore, also, that Holy One who is to be born will be called the Son of God.’” Mary’s question to Gabriel may APPEAR to be similar to that of Zacharias and thus one of unbelief, but when Zacharias said, “How shall I know this?” he was asking for A SIGN to be given to him SO HE COULD BELIEVE. When Mary said, “How can this be?” she wasn’t questioning WHAT WOULD HAPPEN, but HOW IT WOULD HAPPEN. In other words, she wasn’t asking because she couldn’t believe what Gabriel had told her, she was asking because she wanted FURTHER ENLIGHTENMENT. We know this is so for if we were to read on, we would see that Gabriel does NOT rebuke her for a lack of faith, but he answers her question by revealing the miracle of the Incarnation of the Son of God by the power of the Holy Spirit. He gave her the enlightenment that she asked for!  (DO)  (615.1)