Two reasons. One our body don’t turn the fat we consume into fat in our body. We like to turn carbs into fat in our body. That’s why we say carbs make us fat. ( when we have energy surplus) Second when we consume more fat our body gets better at using fat as energy source and start using fat from our own body for energy more efficiently. Meaning we will use more fat when we consume fat. Thus lowering our body fat percentage. (The total size or mass of fat cells will decrease while the number of fat cells don’t change once we are grown ups unless we do diet. That’s like the only way to increase the number of your fat cells so don’t do it. Fat cells almost never decrease in numbers but can decrease in size. That’s why we need to eat healthy from a young age, never diet for long time( intermittent fasting is fine but don’t don’t it when you are still growing meaning childhood and adolescence or just under 18.