In my opinion THIS BIG COMPUTER keepeing cabin rate around default 350 ft/m
You're confused here with the 'Repressurization segment' which occurs at higher altitude at very low weight. In QRH 4.12 (ALL ENGINES DESCENT) you can see that N1 is higher than idle to cater for this segment.
Basically there are three type of segments calculated by the FMGS for the descent profile:
-repressurization (very rare)
-geometric (in between two altitude constraints)
-idle
THR IDLE is normally displayed on the FMA when on the 'idle segment' under forecast conditions.
The 'steepness' of the idle descent is a function of the IDLE FACTOR on the A/C STATUS page.
Especially at higher altitude the 'calculated idle profile' requires more than idle thrust because it is conservative to allow for anti-ice and/or shortcuts. Especially on the A319 this is noticeable.
My thoughts are that Airbus intends to have a uniform type of descent planning/calculated idle segment for the whole A320 family. The A319 is typically lighter than the rest but does fly very similar Managed DES speeds meaning a worse L/D ratio than the rest of the family flying the respective speeds. (i.e. flying further away from green dot than the rest). Hence requiring some additional thrust when on the 'idle segment'.
Sorry for the lack of references, there aren't any. Hope it helps.
S.