GW and CG values are computed from entered ZFW and ZFWCG corrected for the predicted FOB and CG variation. When the Approach phase is activated, the characteristic speeds are recomputed using the actual weight and CG.The performance model used to compute the characteristic speeds, is accurate enough to provide speed errors of less than ± 2 kt from the certified speeds.
I believe that although this modification is incorporated in the latest FAC standards (or was it in an FMGS update?), it's not mandated for retrofit, so a large fleet will have varied setups. Nevertheless it's obvious when the aircraft is slightly slow and I don't think it's unreasonable to increase the speed slightly in those circumstances.
Airbus have to cover themselves from a regulatory point of view and their guidance is just that, guidance. On a 3000m runway there is little risk in increasing Vapp by 2 knots, provided it's taken into account in the performance. Doing it routinely because your landing technique is wrong is a different matter.