In take is the kcal of the food times the digestion factor. The kcal out is the metabolic need, which depend on factors such as fat building vs energy usage, which in turn is influenced by stress but also things like dieting.
This is why counting calories can be an ineffective strategy for some while for others it is a good match.
Let put this is numbers for illustration purposes.
Let say 1000 kcal in, and for an average healthy person 1000 kcal in metabolic rate.
For digestion, this person eat mostly proteins resulting in about 20% of kcal going into energy for the digestion system itself (proteins is relative hard to break down). Let also say that in addition, sloppy digestion and incorrect gut bacteria is resulting in additional 20% of the food never getting digested and resulting in feces. Result is a loss of 400.
If thats the whole story you have a person that is loosing weight despite keeping a diet that seemingly fulfills their calories need.
Lets now change metabolic aspects. Rather than suffering from bad digestion they are suffering from chronic stress with high cortisol, adrenal and other stress hormones. The body crave more food that cost less to digest, ie fat and sugar, insulin is spiking in order to get the body to take up more energy, the body goes into lethargic state in order to save energy, muscle and organ cell production is decreased in favor of fat cells. Instead of 1000 kcal in metabolic rate we might have 600 kcal, with now a surplus of +400 going into fat production.
The upper bound on bioavailable kcals is still 1000 kcal, but the numbers won't tell you if the person is going to loose weight or gain it, unless the person is in good healthy to begin with.
In take is the kcal of the food times the digestion factor. The kcal out is the metabolic need, which depend on factors such as fat building vs energy usage, which in turn is influenced by stress but also things like dieting.
This is why counting calories can be an ineffective strategy for some while for others it is a good match.