The fmgc calculates deceleration to the next speed restriction (i.e. 250 kts at fl100) based on the vertical speed you select. If you select a high VS the 'deceleration segment' becomes longer and if you select a low VS it becomes shorter.. Thus if you select a high VS the managed speed target will come back to next restriction long before the actual waypoint or pseudo waypoint because the fmgs calculates that your deceleration will take a long (if not infinite) time/distance at current VS. Hope that makes sense.