ANP Jumpin Jack Flash is correct. The FMC data set, algorithms and rounding yield very imprecise answers. In comparison to an industry standard flight planning system with accurate CI information the FMC is like a basic calculator vs. a top of the line quad core PC.
We must remember that the key in fuel conservation is the repetitive savings over time. Save 30 kg a flight, for 600 flights a day and well you get it.
The FMC is another flight deck tool with lots of limitations. Its abaility to out perform a flight planning system is a non-starter and using it for precise fuel savings analysis is next to useless. Optimum altitudes, climb/descent/cruise fuel savings values are inaccurate for all the reasons cited.