The bottom chain has twice the load on it and it's exactly the same, except shorter, as the cam chains. That's why it's the one that wears. I found the same thing in the 4.7 I did a chain job to.

Take care,
Rick
68 Coronet R/T 440 & 68 Charger 528 Hemi,and 5 Challengers! 6 cyl, 318, 360, 383, 451