It is just a communication error between computers... On the ground the pilots are the source of ZFW + ZFWCG information for FMGEC's and therefore the FCMC's, which send the GW values back to FMGEC....so it is important to cross check the correct value each side.
Once airborne the FE portion of the FMGEC does some smart calculations to cross check the GW values and warns you if there is an error.
These glitches happen - the important question is what should you check?