The way I understood/presumed it to be (not being a software engineer, it is pure speculation on my part, apologies if it is rubbish), is that it comes down to this:
during the initialization/re-initialization process, the status of ZFW/ZFWCG sent by the FMS differs between side 1 and side 2 during more than 2 seconds.
But, the setting/resetting of the high position for this boolean is not synchronized between both FMSs
So, let's take it step-by-step:
-Presumably, prior to a ZFW/CG entry in INIT B, each FCMC stores either a blank or a '0' value for this data. Let's call this value 'x'
-A 'boolean' setting tells the FCMCs whether ZFW/CG data (let's call this ZFW/CG value 'y') from the FMGCs is being sent to them for their initialisation. If this 'boolean' setting is signalling an initialisation, the respective FCMC accepts it as given and stores the new value.
-If the 'boolean' status is not signalling an initialisation, because, according to the quote above, it is not synchronised between the FMGCs, (the following is pure speculation on my part) as a safeguard, the respective FCMC does not accept the new value, e.g. because it could be a spurious/uncommanded signal.
-So, in one of the FCMCs, the previously-stored value 'x' does not equal the newly-sent, but rejected, FMGC value 'y', which triggers the message.