No Airbus-experience here, but this is how it works on the MD-11 and I'm guessing it should be much the same:
/FF+FQ means that the FMS bases its indication of FOB on two information sources: one is the measured fuel quantity (FQ), the other is the fuel flow (FF - the calculation starts off with the total fuel on board before engine start, and then substracts fuel based on actual, measured fuel flow).
/FQ means that the FOB-indication is based only on measured fuel quantity. This could happen if e.g. a fuel flow sensor is u/s, or during or after a fuel dump situation.
/FF means that the FOB-indication is based only on the fuel flow-calculation. This could happen, you guessed it, if e.g. a fuel quantity sensor is u/s.
The crew can enter /FF or /FQ to force either way of calculating FOB. Clearing the line will return the calculation to /FF+FQ.
If /FF is used, there is NO WAY for the crew to enter the fuel flow! The system uses actual, measured fuel flow. Think of it, it would be nonsensical to manually enter a fuel flow. (Don't confuse this with the fuel flow factor, which can be entered into the FMS elsewhere and which corrects predictions for high/low consumption of the particular aircraft).
So, this FMS ended up in /FF for some reason (and I'm thinking it gets reset to /FF+FQ at the beginning of each flight cycle, so it must have happened during this flight?) but the FMS should still be able to make accurate fuel calculations and predictions!
Crucial to me is: why does the FMS think that the aircraft will land with more fuel than it thinks it has on board now?
I can't think of anything else than a software bug, where the FMS stops updating its fuel predictions for some reason. As suggested by ElitePilot, modifying the route might force the FMS to re-calculate its predictions again.