This is because they rely on static models of weight loss rather than dynamic models that account for the way the body adjusts to reducing food intake by reducing energy expenditure.
When a 20 percent soda tax is recomputed according to a dynamic model, a 20-pound loss after five years turns into a four pound loss (assuming, that is, every other assumption in the tax model is also correct).
Because this static weight-loss rule does not account for dynamic physiological adaptations that occur with decreased bodyweight, its widespread use at both the individual and population levels has led to drastically overestimated expectations for weight loss.