it would also be fruitful to note that you are also comparing 3 distinct genres of music - classical, classic rock, and contemporary heavy metal. if you think about it, you wouldn't be comparing a trance track's mastering volume to a Wes Montgomery jazz song would you? Also I took a look at my copy of the CD and noticed that the mastering guy from the Slayer album is Vlado Meller, the very same guy that has been criticized for being one of a handful of mastering engineers who master music at very loud levels. He also did the RHCP's Californication which, people claim, to have the same affliction.
However, I did a search of Vlado's other work and he did the Eden's Crush CD, Popstars, as well. I actually know someone who owns this CD and borrowed it. The mixing is different, as expected, given the fact that both artists are from completely different genres. Still, the mix between both CDs bear a similar quality of emphasized clarity and punch to them. Maybe this is Vlado's Mixing style?
Personally, I think mixiing, like everything else in music, falls right down to how the mastering engineer chooses to do things. If you think about it, this is really why specific producers go to hire specific mastering houses or engineers - for the sound they can give a given set of songs.
What would be interesting - and a more non-objective way of approaching this analysis - is to take a single mastering engineers selection of works from the same genre and see if he mastered them all a different way.
