There's definitely a heat-loss phenomenon - look for skinny kids in shorts in 40 degree weather - but it's also the case that a variable amount of input calories can be discarded without full digestion by the body - energy not even extracted for use. The religiously calories in : calories out folks assume a linear digestion efficiency relationship between total calories consumed and that this holds unifomally across the population. Given the complexity of biology, they should be unsurprised that there will be myriad outliers