I think it would be fair to make the regulation either way, so long as they put 1kg flour in the bag.
I'd imagine the trading standards would only check to about +/-0.5% (5g); whilst the flour on the bag's surface is probably around 1g for a kilo bag (1kg)?
Well come on, imagine flour was a bit more sticky and if you put 1kg in the bag, only 900g is usable because the rest sticks to the walls. Would that be 1kg bag of flour or 900g bag of flour?