In the US there are very strict rules about labeling something "reduced Fat" or "low fat." Reduced fat means must be 25% less fat than average, and Low-Fat yogurt has 30% fat calories total. When processers take out the fat, they made up for the mouth feel by adding a water/cornstrach mixture and stabilizers. Cheesemakers would use skim milk and a thickener instead of whole milk.
Getting rid of fat is pretty hard to do, so companies use the "light" label to trick people into thinking it's low fat. There are no restrictions on the words "light" or "natural." I've heard that "lighter" sometimes refers to the color. Or they would say "low in
saturated fat" and make up for it with cheaper unsaturated canola oil.
Labeling requirements here:
9. Appendix A: Definitions of Nutrient Content Claims
Of course only a dedicated foodie with a lot of time on their hands (ie no kids) would look at every label like this.
However, if a low-fat diet was stealthily high in fat, then why aren't all these SADs feeling satiated on lower total calories? Why are they so hungry for carbs? Is it the wheat addiction, or maybe these funny fats don't cause satiety like animal fats do?