because it's not all about calories. and calories in/out isn't a set thing anyway... what you eat affects how many calories you burn by affecting metabolism on a cellular level.
and calories in/calories out does not explain how i couldn't lose any more weight on a 1200 calorie/day diet while exercising (low fat, high carb)... but when i switched to primal and started eating more fat, fewer carbs, and a lot more calories (1600-2000+)/day, i lost 6-8 pounds in the first couple weeks. not exercising at all (recovering from surgery).
it's so much more complicated than calories in / calories out... and you have a LOT of reading to do if you are honestly curious! Have fun clicking around!