OST Anthology: Disc 4 - why does it sound so different?

I just noticed that Disc 4 of the soundtrack anthology sounds different somehow. I'm not talking about the actual music being different, but just how it sounds. Was it recorded differently? I really like the way Disc 4 sounds.

 

It was recorded at three different times. Namely the times when each films' score was recorded. The four discs were all mastered by the same person over the same period when the Anthology was produced. I don't think disc four sounds different at all. I think the tracks from ANH sound like ANH, the tracks from TESB sound like TESB and the tracks from ROTJ sound like ROTJ....

 

FWIW, when I listened to the ROTJ disc and then Disc 4, the way the ROTJ tracks sounded in Disc 4 is different than on Disc 3. Disc 4 seems to have better distribution among all the frequencies whereas Disc 3 had more pronounced highs. It could be my imagination, I don't know. What's a good way to determine if the sound spectrum is really different (quantitively)? Run a Fourier analysis?