The simplest way to understand this is that as you lose weight, your body compensates by expending less energy, thus slowing your rate of weight loss. Another way of looking at this is that weight loss becomes progressively harder to achieve the more you lose weight.
Instead of a static model, we need to model weight loss in a dynamic way that captures the body’s processes of physiological adaptation. To illustrate their point, they take one of the most recent and optimistic soda tax studies from the US Department of Agriculture (Smith et al. 2010).
Good point on human body adaption to change in diets and exercise