Weight gain has nothing to do with calories in calories out. It is controlled by blood sugar and insulin. Read the book "why we get fat" about how our bodies work if you want to learn more.
answer: because his money in was greater than his money out.
doesn't really explain much, does it?
specifically, calories in/out doesn't explain: satiety, insulin resistance, muscle anabolism/catabolism, or cravings.
there's much more to it then in/out. yes, thermodynamics applies. no, humans are a bit more complicated than a terrarium.
Okay, I challenge you to eat zero calories and still maintain weight.