Some profiles give wrong fuel calculations.
In EFASS menu, go to Aircrafts, online database (cloud). In the search box, just put IXEG, and look for IXEG 737-300 Working Fuel Calculations by a guy named Maturesh (thanks a lot dude)
This one works fine for me!
BTW if you're not happy with the fuel calculation, for example with the reserve fuel, you can edit the amount. Go to Edit Menu and Customize fuel.
Change the values... and good to go!