I watched Moth3r's DVD of ANH recently and although it is less noticeable, the changes in quality between Leia and Tarkin are also present here. It has always been that way. It is only because of the enchanced audio clarity on the DVD that it sticks out like a sore thumb. Who knows what went wrong? Damaged reels? Bad recording equipment? The fact is that the problem has been present since 1977 and this would probably indicate that it can't be fixed.

As for why Lucasfilm seemingly rushed the DVDs to get them done, read my theory about it in the OT vs SE forum.