All you need is one mathematically challenged person in this chain to have problems with your weight and balance information.
Add this to the inherent difficulty in doing the sums to a reasonable accuracy in many cases and you end up with an unacceptable delta after a while.
Hence, the sensible owner reweighs every now and again to clean the slate - regardless of what the local rules might mandate.
Mind you, unless the folks weighing know what they are doing, the weigh itself can introduce significant errors, especially if one is using jackpad loads cells.
Indeed, if the folk are out of their routine patch, quite silly errors can come into play. Years ago, a local chap over here had rebuilt a Stampe and wasn't able to contact me for a weigh. Not unreasonably, one might think, he contacted one of my airline colleagues to fill in. Now, the airline chap, used only to weighing heavy iron, thought no more about the differences and proceeded to weigh the Stampe with a set of 50k lb Cox and Stevens cells (as I recall).
Now, keeping in mind that the tail load is something in the order of not many kilos - TOTALLY incompatible with high range cells - my owner mate subsequently frightened himself witless on the test flight. When we reweighed on platforms, the empty CG moved forward about 4 inches as I recall.
Lots of (generally simple) traps in weight and balance work.