We are now getting into underlying concerns.
(a) practical engineering - the certification work up activities are based on rational accuracies
(b) certification - the final AFM/POH has to declare numbers which are reasonable results determined from the rational certification work up. However, being written down numbers, they are black and white
(c) legal - make sure that the load sheet, etc., does not disclose a weight figure in excess of the black and white figure in the AFM
Does it matter all that much if the actual weight is a few kilos up or down compared to what you might think it was ?
(a) for the flight, of course not (engineering hat on)
(b) at the Inquiry following the mishap, one had better believe in black and white because the legal folks tend to play that game.