Because we are all metabolically unique. Not everyone processes foods the same way. Some of us metabololize carbohydrates slowly and some quickly. Either of these tendencies, along with countless others, can result in weight gain. The key to weight management is to achieve metabolic balance and which combination of nutrients (i.e., fat, protein and carbs) needed to achieve this state is different for everyone.
The key is to determine which foods and combinations of foods are ideal for your body and metabolic type. The best way to do this is to keep a diet journal. (See my article on “What is A Healthy Diet” to learn how to keep and use a diet journal.) This will enable you to see and feel how your body responds to certain foods and combinations of foods so that you can optimize your metabolic state and burn fat more effectively.