it’s not that simple. your diet can affect how many calories your body burns. in my case i dropped to eating ~800 calorie diets and i actually lost weight slower (presumably because my body went energy conservation mode?)
It really is that simple, as you said in your post. Your metabolism (rate of burning calories) is not constant. Your actual consumption of calories versus your actual burning of calories dictates weight gain or loss (barring degenerative diseases).
If you are 800 calories, and burnt 1100, you would lose weight slower than if you ate 900 and burnt 1300.
If you are 800 calories, and burnt 1100, you would lose weight slower than if you ate 900 and burnt 1300.