I've learned that if you eat a food item that is harder to digest, it ends up lower in calories than a piece of food which is soft and malleable.
For example.
Flapjack (hard)
Advertised calories: 200
Actual digested calories (non-scientific guess): 180
Chocolate Brownie (soft, yielding, chocolatey flesh...)
Advertised calories: 200
Actual digested calories (non-scientific): 200. Or is that 210? Hmm...I need to get back to the article and check my facts...
Anyway, that's all roughly true. Don't ask me why. Ask the New Scientist.