Because the calories leak out. This is a well-known fact. But I'll bet you were unaware of the many other amazing calorie loopholes in our diets today! For example, did you know that if a friend makes some dessert, and you eat it so her feelings won't be hurt, the calories never count against you? And if you eat something quickly and furtively while on the go, the calories forget to show up! And then there's the loophole I exploited just last evening: if you eat food in order not to let it go to waste, it won't go to waist.
What happened was this: all the huge, gloriously red-gold peaches on my parents' tree suddenly turned ripe in one weekend. They dropped off the tree and had to be harvested, but I couldn't keep up with them all. There was nothing for it but to cut them up and make peach crisp to take in to work today. And then of course I ate half of it myself. But in this economy of ours I simply couldn't think of wasting food. :)
That, incidentally, is why I've also been eating zucchini pancakes all weekend. A co-worker has a tree and gifted me with a huge zucchini on Friday.
Well, I know where I get all this from. My mom in her pre-skinny days carefully followed the calorie-discount rules. She'd take a whole unopened package of cookies in her hands and say, "Now, I'll only eat the ones that are broken." Then she'd SLAM the package down on the counter.