Oni When Ann was a child, she learnt the alphabet with a Bible A to Z book (eg. 'A is for Adam'). At the end, there was 'Z is for Zachariah'. As Adam was the 1st man on Earth and was at the beginning of the book, she believed that Zachariah was the last man on Earth. She only believed this at the time when she was learning the alphabet, though.
This answer contains spoilers… (view spoiler)[Ann had a Bible A to Z book and A was Adam, the first man on Earth. Z was Zachariah, so she assumed that Zachariah was the last man on Earth. Since Mr. Loomis was the last man on Earth (according to her knowledge), he receives the title of Zachariah, hence the name "Z for Zachariah". (hide spoiler)]
Trevor Winter when ann talks about learning the alphabet from a catholic book, she say's that z stood for Zachariah, she asumes that Zachariah was the last man
Bobby It was an analogy of Z being the last letter in an alphabet back, standing for Zachariah and her thinking she was the last person.
J My theology isn't the best but i believe it is suggestive of the relationship between the biblical Zachariah, John the Baptist and the deciples of John. But you would need someone with an understaning of these verses and stories to give you a fair compairison.