I can't think of any reason why the two engines should behave differently.. how big is the difference?
Incidentally "flight idle" does not exist on A320, it is modulated idle with Flap lever at Zero, idle varies with bleed setting.
Approach Idle uses a/c altitude and Flap lever position - so on a clean a/c how do you know which applies? Answer - I certainly don't!
Perhaps the FADEC's disagree slightly..
Edit: I assume you are talking Simulator - have you told the engineers?