The accuracy of stated energy contents of reduced-energy restaurant foods and frozen meals purchased from supermarkets was evaluated. Measured energy values of 29 quick-serve and sit-down restaurant foods averaged 18% more than stated values, and measured energy values of 10 frozen meals purchased from supermarkets averaged 8% more than originally stated. These differences substantially exceeded laboratory measurement error but did not achieve statistical significance due to considerable variability in the degree of underreporting. Some individual restaurant items contained up to 200% of stated values and, in addition, free side dishes increased provided energy to an average of 245% of stated values for the entrees they accompanied. These findings suggest that stated energy contents of reduced-energy meals obtained from restaurants and supermarkets are not consistently accurate, and in this study averaged more than measured values, especially when free side dishes were taken into account. If widespread, this phenomenon could hamper efforts to self-monitor energy intake to control weight, and could also reduce the potential benefit of recent policy initiatives to disseminate information on food energy content at the point of purchase.
OK, there are many reasons. And I'm not a calorie counter, but really I avoid processed foods because I have a very fuzzy intuitive sense of what I'm putting into myself. If I took the time to analyze the labels in detail, it seems clear that there's going to be a lot of error. This isn't a surprising finding, it's been reported before. It makes common sense, as the key economic driver is going to be be making food taste better and cheaper, not reducing the measurement error baked-into-the-cake of their labels. And no surprise that the measurements are erring on underestimating calorie-counts.