Inaccuracy!
As with my limited experience on the NGs, I have noticed the Estimated Fuel Remaining Over Destination, progressively reduces by approx 500kg through the flight, without any other attributible reasons. So if you see a figure on the PROG pg at the beginning of a flight, it usually changes by approx 500kg by the time you get there. This compared to the 744s, which was definitely far more accurate than the 738s.