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.

Or maybe--just maybe--there's no explanation for it all, in which case I still don't really care. It's not a big deal. It's a different movie, shot at a different time and by a crew with fewer resources and experience than the one that worked on its sequels. There is no issue with the costume, and none of the (brilliant) shots in which it appears need to be reshot.

Just my two cents =P