That’s assuming perfect conversion of energy to body fat. That’s never the case due to losses, and everybody is _vastly_ different:
> In response to 84 days of 1000 kcal/day of overfeeding, 12 pairs of monozygotic twins gained on average 8.1 kg, but the range was from 4.3 to 13.3 kg.
Apologies. Though my statement holds with s/Your/That/.
My original response was to a comment suggesting that overweight and obese people get home and eat 15 chocolate bars. My observation, supported by your additional evidence, is that a small imbalance has a substantial cumulative effect.
The study was with 12 pairs of identical twins. Variation of 25% in IDENTICAL TWINS suggests that variation among genetically different individuals is probably going to be much larger.
> In response to 84 days of 1000 kcal/day of overfeeding, 12 pairs of monozygotic twins gained on average 8.1 kg, but the range was from 4.3 to 13.3 kg.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2897177/