according to FSX, the empty weight of the J41 is 14,144 lbs.in the FMC, it's different (it's 14,600 something). i find i can manually change it in my PLAN page but why is it different to begin with?

What's the EOW of a Cessna 172?Google it and you'll find more than one answer. Simple answer? It's rare that you're ever going to see an aircraft weigh exactly the same as another. The FMC likely uses an averaged base weight. Look at a chart from the FAA that shows runway length, and it's likely rounded as well (except larger airports that have been surveyed by the FAA). Aviation is very dependent on numbers, but the science to the madness is that those numbers are seldom exact.Another pointer for you: "according to FSX" is generally the lead in to something being wrong. Example: According to FSX, precipitation is a simple effect that follows you around wherever you go.