sfb:

Why does exercising when unfit burn fewer calories? Is it because you cannot exercise as intensely?

Pretty much, both when it comes to the maximum power you can exert and the time you can sustain it. To take a very extreme example, Michael Phelps can exercise intensely enough to burn almost 10,000 calories in a few hours every day (of course, there's a large genetic component to it). An average couch potato would likely collapse before managing to burn the equivalent of a single Starbucks muffin in a stretch.