I think it is the fact that F speed is "about" 1.18 VS to 1.22 VS.
They don't explain why the non-exactitude of F speed, but I would say that is the reason.
As for Sebxl post, the FMGC calculates the speeds based on estimated landing weight until approach phase is activate, from which point actual GW is used. he is right, I htink
anyway, F speed will change at about 1 or 2 kt per ton, maximum. The difference for that reason should never be 15 kt!