Yeah, all three of those explanations has been given at some point in the EU. Never in any movies, though.
I guess it was a way to explain why it only took one shot to down a TIE and a few to take out an X-wing, since Wedge and Luke both get shot and don't die and because they're mentioned as having them. Not that it's a terribly important distinction to make, but it adds a little colour I suppose.