sleemanj: Ok, further analysis with a second box of the same Double Choc;
Average: 39.79g (Gross)
The wrapper itself is fairly consistently 1g (and yes my scales can measure down to 0.01g (but lets say 0.1g reliably to be fair)), so we could guess a net weight of 38.79g average. Still less than the 40g advertised in big print.
And a box of "Nutty Crunch" (again Tasti 40g);
Average: 44.72g (Gross)
a) The first box of Double Choc must have been very unlucky (yes, I re-weighed it along with the others, the bars are also clearly about 1cm shorter than the others which explains it)
b) Tasti's measuring equipment is really not very accurate! You wouldn't think it would be too hard to produce a product that was within a gram either side of a target :/
c) Don't trust advertised product weights further than you can throw them.
d) I shouldn't need to buy snack bars in a while
How accurate are your scales though. Are they proper scientific ones which have been audited, or are they just general consumer ones, which would have a margin of error.
One thing I have noticed is that scales in the supermarket, which you weigh produce on and get the prepinted price ticket, can be out by quite a bit, and they seem to overweight, so you pay more, especially high priced produce. So it never seems to work in the consumers favour when this sort of thing happens.