Models are always approximations; they are never spot on but will get you in the ballpark. Even if calories isn't perfect it still reinforces the notion that reducing your intake will reduce your weight gain.
There is no Batman if there is no night time.
"I am the damp-dreary-overcast-day!"
A successful [software] tool is one that was used to do something undreamed of by its author. -- S. C. Johnson