Rat5,
The exact limitation difference is for AUTOLAND WITH AUTOMATIC ROLL-OUT; 15kt vs 20kt for non-sharklets aircraft.
When you hand fly an approach in gusty crosswinds, you need to 'fight' a little more with the aircraft to keep it on the centerline. This is due to the added vertical surface area of the sharklets blowing the aircraft to one side.
I guess, Airbus felt that the NWS and flight control systems weren't up to the task once the aircraft was on the ground, hence the limitation reduction.