The way I look at it, there is no ZFW nor ZFCG computation. They are always pilot entered. There is no way that those values differ from FMGC computed ones because the FMGC does mot compute any ZF values.
The FMGC only computes GW values (and TO and LDG) bases on ZF and block fuel pilot entered inputs
Now, if there is a difference between FCMC computed GW and/or GWCG (which use pilot entered ZFW, ZFCG and FCMC sensed fuel quantity) and FMGEC's (which also use pilot entered ZFW & ZFCG)... Why does it trigger a ZFW/ZFCG discrepancy? It is clearly a fuel related discrepancy, that of pilot entered block and sensed quantity, and even the ECAM is a FUEL one.
Is there any ECAM alerting from a difference between FCMC and FMC fuel quantity?