Why is Nancy short for Anne?

There are many instances in English of the letter-sound "n" being added or subtracted. For example, "uncle" was for a time "nuncle" and "apron" was "napron" (notice its resemblance to "napkin" now). It's supposed that the confusion relates to the fact that we use "a" as the article for Words that begin with consonants and "an" for those that begin with vowels, so there's not much difference in sound between "a nApple" and "an Apple."