I do not want to edit anymore.
I just checked manual and there is written that FOB is boxed amber when the quantity shown is not all usable. So aircraft knows it can not use all the fuel but still measures all of it.
Then there is written that MCDU uses FOB so it could be tricked by unusable fuel.
Is this correct?
We had recently problem with B777 and since that they require extra information about this "mess" with ZFW. Maybe Airbus is more inteligent but that is the problem. We do not know. Some people do not doubt but I do.