"The idle segment assumes a given managed speed flown with idle thrust plus a small amount of thrust. This gives some flexibility to keep the aircraft on the descent path if engine anti-ice is used or if winds vary."
Funnily enough this is the concept of THR DES on the A330/340. This mode does not exist on the A320 but essentially does the same thing.
However, have you ever noticed with THR IDLE on the FMA, and the thrust slightly above IDLE on the E/WD, the aircraft tends to accelerate to the +20kt tolerance and only then trust is reduced, and thrust IDLE on the E/WD.
Maybe your forecast to fly into an increasing headwind on descent so it makes an additional allowance. I don't think it's that clever but you never know.