I believe that Zacherias was disciplined because we can detect a spirit of doubt in his question to Gabriel. "How shall I know this? For I am an old man, and my wife is well advanced in years." Zacherias was a priest that served in the Temple and should of known full well that nothing is impossible for God. Mary's question was one of a young teenager with a limited knowledge of pregnancies, so, I believe her question was not of Gods ability to perform this miracle, but of how He would do it. In short, she was in awe of the Lord her God.