The first thing we need to consider is bio-availability. Bio-availability is the term used to describe the amount of calories that are actually absorbed by the body. For example roughly 90% of the calories from carbohydrates can be absorbed, whereas only 75% of protein calories can be (protein burns off more calories being metabolised than carbs). This is one reason why low carbohydrate, higher protein diets are more effective
The next thing we should look at is how many calories do we need from carbs, fats and protein. An average female, needs roughly 1500 calories a day to stay alive, this doesn't include any movement they do and all of this energy needs to come from fat and protein, as carbohydrates can only be used for movement. If this person was focusing only on calories and not nutrition, then they may consume 800kcal of carbohydrates, 400kcal of protein and 300kcal of fat. This would mean they would be nutritionally deficient to the tune of 800 calories every single day, resulting in muscle wastage (so they may lose weight on the scales , but it would be muscle, not fat), increased risk of heart disease, reduced immunity and fatigue.
So what happens to the 800 calories from carbohydrates?
Like I said earlier, carbohydrates can only be used for energy. A moderately active women may use around 500 calories during a day, if she wasn't partaking in any strenuous exercise. That leaves 300 carbohydrate calories, and the only other available option for these is to get stored as fat. By eating an appropriate number of calories for fat loss, this lady has managed to be nutritionally deficient and gain fat because she thought a calorie was just a calorie.
I would not recommend anybody count calories, it is too inaccurate. Plus most of the foods you should be eating don't come with the handy nutritional breakdown on the back. People who count calories also tend to be the ones who try to measure how many calories they have burnt in the gym. The counters on treadmills and apps you can get on the internet are guesses, even the heart rate monitors are only accurate to 10% either way, which could put you 160 calories out in one workout! There is far too much guess work for it to be a reliable method of losing weight.
Eat real food, lift heavy things and walk.
It doesn't need to be any more complicated than that.