Okivan;
Can you find out whats wrong
Actually, I wouldn't be interested in what's "wrong" with the FMS.
The FMS system is for predictive estimates, not for official numbers.
The question is rather, what does your OFP - Official Flight Plan state as the FOB over destination? That's the number that governs the operational viability and even the legality of the flight.
The FMS is a planning tool that may not guarantee 100% accuracy but the OFP is expected to be accurate, especially where it concerns fuel on board over destination .
If the discrepancy is large, by all means have a look to see what the problem is and resolve it to the satisfaction of all on the flight deck. For me, 500kg isn't a large discrepancy to see, even in an A319 FMS especially when we don't know the source of the difference and therefore can't know which one is correct.