FMC descent
The FMGC calculates the descent path with the ASSUMPTION that anti-ice will be ON.
A you should know, having anti-ice ON increases the demand for bleed air, and in order to be able to deliver that amount, the engine has to run at a slightly elevated level of thrust (that means, a little more than idle).
If, as often happens in real life, the descent is made with anti ice OFF, then idle thrust will not be enough to hold the speed constant, so thrust will be kept slightly above idle by the autothrust. Remember that in descent, pitch mode will keep the aircraft on the descent path.
When you use open descent, pitch mode will steer to maintain speed, while autothrust goes to idle.
The assumption of the FMGC is programmed to keep you from f***ing up your approach: if it was planned with real idle and you needed to employ anti-ice during the descent, you would end up with too much energy at the end of descent and that has proven, on a number of occasions, to be too much to handle for the magenta generation.