The WABC (Weight And Balance Computer) system is ano option on most Airbus and Being aircraft. SOme of the 744s I used to fly had it. But we always ignored the WABC figures and went for the loadsheet figures. Flying out of JFK with a couple of hunderd 'average' Americans on board, with their "3kg" of hand baggage often showed the latter's futility. I have seen a TOW indicated on the WABC some 20 tonnes (yes, 20 tonnes!) greater than the loadsheet. And I have also seen the WABC say we were several tonnes over MTOW once.
The Airbus FBW aircraft measure the weight when airborme through the Flight Augmentation Computers (FACs) from data provided by the angle of attack. Our A321s are always a couple of tonnes out, always heavier. And from the way the aircraft performs, I suspect the FACs are correct. The FACs provide the protection speeds wheras the FMC provides the approach speed from the loadsheet weights input at the planning stage. Adding a few knots to Vapp is the norm.
PS. The last time I was notional weight, I was age 16. Time to change the notional weights methinks!