The number of calories that go into your mouth can be very different from the number of calories absorbed by your body. To give a simple example you'll absorb

The number of calories that go into your mouth can be very different from the number of calories absorbed by your body. To give a simple example you'll absorb way more calories by eating a piece of bread with olive oil than if you eat the two pieces of food a couple of hours apart. And someone with intestinal parasites will receive notably less nutrition from their food than someone without. It would would actually be very surprising if intestinal flora didn't effect the ratio of calories consumed to calories absorbed.