Normally the BLOCK fuel is already there. Then we enter ZFW/ZFWCG and finally
get the message.
The time it happened to me, we had done exactly the same.
I'm fairly confident that it's something to do with the difference between the actual FOB and the pilot-entered BLOCK figure in INIT B, and the consequent difference in the result of a particular calculation that the FCMC and FMGC each do. I just can't, for my life, remember the precise definition.
One thing that doesn't help is the lack of (clear) information from Airbus regarding this - it's just a case of us knowing the logic that the computers use and what is done with what figure. Cue the discussion that the airplane is a flying computer program.
