I don't know the full answer to your question, but I believe that food companies are allowed to round down when the amount of a given macro-nutrient (fat, protein, carbohydrate) is low enough (less than 1 gram, perhaps?).

That might account for the 'missing fat' you're talking about. To me it feels a bit disingenuous on the part of food companies, since it presents the image of being meticulously accurate (1/2 a gram is such a small quantity), while simultaneously glossing over details.