Firstly for the other ME forum readers this is an EK question
One reason is that FABS2, just like the online check-in numbers, does not always (ever?) include transit pax. It's numbers are good at the flights origin, say DXB but if for example you check the loads SYD to DXB on the 413 it does not include those in transit from AKL and you have no way of knowing that number with FABS2. The best bet is Apollo for a transit station, it's on the FOIP.
As for the TripsNet figures being reliable, ha ha ha ....