737 NG FMC Inaccuracy

The FMC does'nt reflect accurate GrWt in the perf init page. For instance, when loading the NG737 700 Northwest Livery the GrWt that appears in the FMC is 69.5( 69,500 lbs )....this wt apprears irrespective of the weights reprsented in the load manager or FS9.If another livery is loaded such as Southwest there could be another value for GrWt such as 134,000 lbs. If I manually change the GrWt the v speeds can't be entered. Is this a known issue with this product, and if so, is there a work around?Thanks,Irv Strongin