Is pilot entered block fuel ever used for any computation?
I no longer have my A330 manuals and I can't seem to find the full set online. I don't want to rely on memory and potentially also mix it up with any A320-specific information that I was also on, so I wouldn't like to give a precise answer, which would be conter-productive, if wrong. In other words, take what I say with a pinch of salt.
Anyway, from what I could find/remember:
"The FMGC computes the FPLN predictions, based on the entered BLOCK fuel, and estimates the EXTRA fuel value." - this is before engine start.
"After engine start, the FMGC will stop using the pilot-entered block fuel and will compute its predictions based on the FOB indicated by the FCMC"
Perhaps the same before/after-engine-start logic applies to the FMGC GW/GWCG calculation? I.e. the BLOCK value is used in the FMGC GW/GWCG calculation before engine start? If so, a difference of sufficient mangitude between the actual FOB and the BLOCK values would result in differing FCMC and FMGC GW/GWCG values and hence the message.
Apoligies if I am talking rubbish here.
EDIT: Perhaps Mr DozyWannabe has some info?