Hang on, if they knew the final ZFW why wouldn't they spot the weight problem then?
It's a while since I last saw a loadsheet, but all the ones I've ever encountered have used a standard passenger weight, which obviously implies a typical adult male/female pax ratio and hence weight (84kg in this dummy example):
Clearly if the actual mix is far from typical (as in this case), the loadsheet won't reflect the true ZFW. I'm not saying that the difference would be critical, but in this instance somebody obviously did think it was.