I can't find the other thread about this, but here is a sum up.
A320 also has a THR DES mode, so to speak, but it is transparent to the pilots.
The IDLE FACTOR in the FMGS determines this higher idle when using DES mode. It is intended to give an allowance for anti ice or give some "flexibility" that is, to give you a few more miles from TOD just in case.
I hate it, because it makes my own computations and FMGS's differ.
Note: the repressurization segment also involves a higher idle but this is to reduce v/s and time to descend so that the airplane can lower the cabin altitude without exceeding the preset cabin v/s (350 fpm default)
Where is this written? I can't remember. It is somewhere in the FCOM"4" and I think there is also an FCOM bulletinq about it