I think where CICO fails us is obviously not in the physics equation, it’s the mechanics. You drive a Civic. You change the oil every three months, you drive like there’s an egg between your foot and the gas pedal, you religiously inflate and rotate the tires, you drive on asphalt. I drive a Civic. I haven’t changed the oil since I bought mine, I drive on sand and I like to pretend I’m Richard Petty. You get 42 mpg and I get 29. Same CI, same number of miles driven, you’ve still got a quarter tank of gas and I’m thumbin’ a ride to the gas station. (I did however have a lot more fun than you did😁).
If you break that down it may all come down to explicable little physics equations - friction, inefficient burn etc. - but the real world difference in performance makes your “mpg” almost meaningless to me and vice versa. The energy in a gallon of gas is merely one factor in how far I can go.
ETA:. Taking this back to human bodies, there are thousands of variables, including some we don’t really know well. To begin with we’re not all driving the same car, but even if we were, maybe you have an optimal hormone profile. Maybe I have extraordinary gut microbiome. You can’t sleep, I have excema. You got a new puppy, I lost an old companion. The shape of your femur requires more energy to move than mine, I chew my food more times. You fidget, I meditate. I’m “fat adapted”, you’re aerobically fit. How any of that affects our real world CO is an unfolding and pretty individual story. Maybe keto or some other tweak is like engaging a hybrid system we didn’t even know we had on board.