FADEC is using Modulated Idle
True. But we're talking about the FMGS commanded THR IDLE which is no engine idle but some sort of assumed idle for the descent profile. (Idle does not flash on EWD when FMA reads THR IDLE).
My bet is on uniformity of descent profile calculations:
e.g. given weight (no exact numbers, just for illustration)
A319 green dot 210
A320 green dot 220
A321 green dot 230
Managed descent speed for all 3 a/c: .77/290
At this fixed speed schedule the A321 has the best relative L/D ratio because it is flying closer to Vmin drag=G.D.. Hence has the shallowest descent profile. This A321 profile adopted for all 3. This means that the A319 and A320 require thrust to stay on this profile.
This also explains that DES and OP DES result in more or less the same profile on the A321 while on the A319 and A320 the a/c will descent below the managed profile in OP DES.
Just my thoughts.
Anyone?