Stolen from Facebook

it’s still calories in calories out

Yeah, but it's hard to count calories in:

• Plenty of foods are inconsistent. One apple might have twice the sugar of another.
• Some nutrients have a calorie count but aren't necessarily metabolized in the same way between people. One example is lactose intolerance, and whether and how to count those sugars. Another is resistant starch or certain oligosaccharides, which can be metabolized by most people's microbiomes but not be used directly by human guts. The precise pathways determine just how calories end up entering the human bloodstream, versus some portion of the calories absorbed and used by microbes, still bound up in chemical bonds as it leaves the body, etc.

And it's hard to count calories out:

• Resting metabolic rate varies based on circumstance, and can change in the same individual based on lifestyle, hormones, etc. Any CICO calculator ends up just trying to infer baseline metabolic rate from rough models of how a certain height/weight/age/gender combination tends to work, but still requires tweaking from watching days or weeks of strictly counting calories and measuring the calories burned from exercise.
• Thermogenic effect of food makes for a certain amount of energy expenditure from simply trying to digest food.
• An individual's own exercise form can determine just how efficiently they can move, or how many calories they're burning from a specific activity.
• Some forms of exercise can increase energy expenditure during recovery.

And all this misses the biggest weakness of trying to use CICO alone as the framework for managing weight: how we feel ends up driving a lot of our behaviors, including whether we'll stick with our diet or exercise plans. Junk food, alcohol, and even non-food drugs like caffeine or nicotine affect our appetites and our exercise fatigue, and do have a real world effect on whether we will actually do what it takes to manage the calories in or out.

For each person who is measuring their calories wrong, there are probably 10 people who just won't stick with a plan. A good fitness plan accounts for this, and works in ways to keep a person on track. And that might look different for different people: forbidding certain foods, encouraging certain foods, meal timing or intermittent fasting, certain sleep habits, hydration levels, managing what foods are easily available on hand, etc.