mike,
RF Fan gave you the answer : the “AMR” subscriber does not disconnect the call while in bad radio conditions. The radio is bad, but the voice is good –> the subscriber is happy and keeps talking. Unfortunately the SACCH are lost, and the call drop will eventually occur anyway.
With non AMR, the subscriber will hang up the call before the BSS could detect a call drop.
Therefore, non-AMR CDR < AMR CDR.