Captain Antilles said:
Call me crazy, but I actually like the so-called "matte-ness" of Vader's ANH helmet; it's kinda cool. If you're looking for an in-universe explanation, then who's to say his helmet doesn't receive upgrades over the years? And maybe those upgrades differ slightly from each other in appearance. Or maybe he has different helmets for different purposes.
Yeah, I always just used that reasoning. It always seemed to make the most sense lol.