I want to say that food manufacturers are allowed up to a 20% error rate in terms of their nutritional information, which makes being accurate very difficult and yet another reason to eat more "real" food and less out of a box.
I noticed something weird yesterday evening. I had a baked potato with dinner. For planning purposes, I entered it raw as weighed in grams, then once I cooked it, I weighed it in grams again...and there was a 10 calorie difference. Not a huge deal, but how did 10 calories get added just by cooking?
I guess it all just goes to show it's not an exact science by any means. I often wonder just how accurate it really is, and hope that the overages and shortages just even each other out.
And now that you don't have to be perfect, you can be good.