FCOM DSC 22-40-40
Above 14500ft VLS is computed out of the gross weight, so if you've not been as accurate as you could have been when it comes to updating the gross weight or FOB, you'll get a slightly erroneous calculation of VLS and hence VAPP will appear closer than 5kts.
Below 14500ft VLS is computed from current angle of attack, speed/Mach, altitude, thrust, and CG. There's by definition an inaccuracy in this method, which Airbus quote as +/- 3kts in config full.
Policy at my previous airline was to add a couple of knots to VAPP to ensure a 5kt split in this situation.