I remember coming across this a long, long time ago. If memory serves me right, isn’t it the case that the FCMC used the pilot-entered ZFWCG and calculates the GWCG by taking the actual fuel mass/distribution onboard on a per-tank basis, whereas the FMGC used the pilot-entered ZFWCG and calculates the GWCG by assuming a standard fuel distribution for any given FOB? A non-standard fuel distribution would, of course, result in a different fuel CG effect and hence, a difference between the GWCG outputs from the FCMC and FMGC.
Also, had you already entered a BLOCK figure in INIT B when the message appeared?
It slowly starting making some sense
Normally the BLOCK fuel is already there. Then we enter ZFW/ZFWCG and finally get the message.