I wasn't talking about fiber, but the sugars bound to fiber. Its very hard to accurately labele just the bioavailable calories, even if you account for things like fiber.
On the note of genetics, it's not just about metabolism. People have different abilities to even absorb the same calories. People have food intolerances, different rates at which they move food through the digestive tract, and different intestinal permeability.
This isn't meant as an excuse to eat junk and not pay attention to your food. But, I actually find more help in paying attention to food quality and listening to how your body interacts with different food. E.g., eat less processed food, be aware that eating fat slows digestion, pay attention to your intolerances, stop eating when full, cut out snacking (again, especially processed foods). If you do this, its very likely you won't need to count at all.
That's not to say that, if calorie counting works for you, then you shouldn't do it. Its just not the end all be all people act like it is. Pretty much any diet and paying more attention to what you eat in any way works: https://pubmed.ncbi.nlm.nih.gov/32238384/
Ok but my point is you're not just eating cake so its hard to keep track of the linear relationship sometimes. Calorie reporting can be incorrect and bodies are weird. That's all I'm saying.
Realistically, being on most any diet is equally effective. From simple calorie counting to the keto diet. It turns out that, if you find a diet you can stick to, then just kind of paying attention to what you're eating in a general sense works.
https://pubmed.ncbi.nlm.nih.gov/32238384/