This is dealt with in the new tutorial in fact. It's a real quirk of the airplane. It doesn't matter in the end because all fuel predictions in the FMC are offset by that same amount - the burn values are what matter and those are always right.

Here's an interesting discussion on the issue outside of the NGX:http://www.av8n.com/fly/fuel-gauges.htm

Yes, as far as ga is concerned I believe the fuel gauges in the Tampico I fly are only required to accurately indicate when the tanks are empty. I don't know if it's any different in airliners.

And the real problem ... when one reads zero, the other reads 200 and it is "very quiet"!