The FCMC calculations are used for CG control and is the primary source for
FM and FG.
The FMGEC values is the backup source for FM, FG and is primarly
used for AFT CG monitoring.
(as watchdog for FCMC CG control)
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?