The difference
I fly A320-214/232. Different speeds on mcdu and pfd are quite common. The MCDU always uses the ZFW you enter. Its calculations are based on that ZFW throughout the entire flight. On the approach phase, speeds like F, S and VLS are estimated speeds. On the other hand, PFD shows FAC calculated speeds which are actual depending on your configuration at the time.
During climb, FAC uses certain thresholds (>14500 feet, >250 kt and some more) to calculate speeds on PFD depending on the aerodynamic model. Then it uses fuel consumption model and during descend it reverts back to aerodynamic model again.
(Sensitivity of these speeds depend on the sensors' accurate working of course)
So if you have a different speed on PFD than the one on the MCDU (on a cyrstal clear day, to eliminate the ice accretion affect on the matter), say different VLS then you are either heavier or lighter depending on the speed being bigger or smaller. If sensors are working properly then it means you're at a different weight than you think you are. We need to be careful on that because CHECK ZFW message appears at 7K! So if there is a difference below 7000 kgs, you will have no alert messages. That can be a big enough difference to think on adding speed to target speed on PFD which uses Vapp on the MCDU!