That's just the way it is. The FMS is in pounds, while the overhead gauges are in Kilos. I don't know how to change it in the FMS so I modified my overhead fuel panel to display the fuel in pounds so it matches the FMS though.

Here is the Topic to explain how to do it: http://www.captainsim.org/yabb2/YaBB.pl?num=1245096833

Mark