They're not being individually tailored. It's almost like being weighed at the checkout and then being told that this is your calorie load.
There has been a very large cross-sectional study in the U.K., looking at doing these measurements relative to height and weight and seeing if eating behaviours are different between children who are overweight and those who are not overweight. They don't actually reveal any striking differences, but there is a major problem in that children who are overweight tend to under-report food intake. Specific types of foods get under-reported. Chocolate is a well-known one for being under-reported, for example.