> Two similar people can consume precisely the same number of calories and have the same level of physical fitness and physical activity and have different outcomes.
You are trying to find a loophole here that doesn't exist. If two people burn the same amount of energy E_O ("out") and take in the same amount of energy E_I ("in") with E_I < E_O, then both of them must take those calories from somewhere else in their body and must therefore undergo weight loss. There is no way around that.
Now, notably you didn't specify E_O, you only said
> the same level of physical fitness and physical activity
which can mean a lot of things.
All the supposed loopholes work in exactly the same way: It might not be entirely clear what the values of E_I and E_O are, so "calories in, calories out must be wrong". (Which, again, is impossible because it's just conservation of energy, i.e. a law of physics.)
In any case, just like you can bound E_I from above by counting calories, there are ways to constrain E_O from below: If both people have the same mass (say 100kg) and walk up the staircase in one of the towers of Cologne cathedral (~150m), then they will both have spent at least E_O >= 100kg * 150m * 10m/s² = 150,000 J.