Most factory food manufacturers derive their nutrition labels from data provided by the supplier of the raw ingredients ... they do not, as most consumers assume, test the final product, or test anything for that matter.
There is a competitive advantage as a raw ingredient supplier to have low calorie numbers as a manufacturer may choose an ingredient supplier based at least partly on nutrition criteria.
The nutrition label can be off by +/- 20% and still be considered accurate by the FDA or Health Canada (the label reporting requirements vary slightly but the regs are identical in each nation).
It's standard procedure to skew your nutrition label depending on the value consumers place on the criteria; in other words you de-value calories and fat by 20% and over-value vitamin content by 20%.
In Canada they had a program to test compliance of food labels; they abandoned it under pressure after about two years ... fewer than 20% of labels were accurate, and roughly half were not even close (200+% error), but rather than being random error, they erred in favour of lower calories, higher nutrition, etc.
As always, use your common sense. Expect labels to be optimistic in favour of the food producer, and it's debatable whether you should choose one product over another over relatively small nutrition label differences (perhaps 25%)..
