Language can be a funny thing. It changes and evolves because it's a living code that we all share. In this case, the word Napron suffered something called metanalysis, a process by which a word is broken down in ways that were not original to it. In this case, the phrase "A Napron" had the N move to the article and it changed into "An Apron."
Another example of metanalysis is the name of a species of snake: a nadder became an adder. There are other types of metanalysis. For example, people took part of the word foremost and have attached the 'most' part to other terms like rightmost, uppermost. Clipping words is also a for of metanalysis, for example, calling it a copter instead of a helicopter.