It's like Bryan says, all in kgs. Based on the flight route, weights, FL, wind predections etc, the plane thinks (at departure), that there will be (e.g.) 5200 kgs of fuel left at destination. As the flight progresses, the plain recalculates a lesser value. It might be that Active Sky uses other wind data then PFPX (in of 10 cases). PFPX provides the wind aloft data for the fuel prediction during flight (only once, at departure if I am not mistaken). Active Sky provides the actual winds that are encountered during flight. In theory they should be identical, but perhaps this is a clue for this mysterious fuel disappearance. Question of course is, if potential differences in wind can explain this excessive fuel consumption. The fact that the issue is happening only 1 in 10, seems to indicated that Bryan is doing nothing wrong. Otherwise, the issue would appear 10 out of 10.