Hi PilotJames,
There is no explanation given in FCOM but if you look at this old
oeb-819-1 it explains on page 3, how the aircraft weight is re-computed and updated for the approach.
Amongst some other conditions provided the dynamic maneuver load is <1.07 and no change in configuration then the FACs will recalculate the weight and hence V stall 1g.
I don't know how it knows there is no configuration change but if it was for say 1 second after the flap handle had been moved, and the flaps ran slowly so a recalculation was made during flaps transit then there could be an error if the maneuver load was <1.07.
Maximum error at 66 tons = 1.07*66 = 70.6 tons (4.6 tons which is about 4 kts change in VREF).
Once stable at 1g, then another recalculation to the VLS would then be correct and reflect the reduction in VLS as the flaps extended.