It is possible that th enuts were produced and labelled in the Europian Union or somewhere which uses the same system as we do. In this case the fibre has already been deducted so you can't do the sum again! Thus the ones which seem to have -4 carbs actually have 3 grams of effective carbs and 4 grams of fibre thus making 7 in total. These figures would also seem to be much closer to the truth.
America, as always, has it's own way of doing things!