Hi dovish,
TTex600 is correct. You probably had a head wind component during the approach which was about 20kts greater than the surface wind value you'd entered in the FMGC.
FCOM 1.22.30 Autothrust
"During the approach, the FMGS continuously computes the speed target, using the wind experienced by the aircraft, in order to keep the ground speed at or above the "Ground Speed Mini"."
We used to do the same maths on 707s using INS "WIND HOLD" for the instantaneous wind component on the approach, and adjusted our approach speed depending on the reported tower wind (exactly as ground speed mini does now). The power only changed to suit the ROD, and not to chase the temporary gusts or wind shear.
It's so much simpler now.