The answer is simple.
Diet soda slows your metabolism through a different mechanism. Diet soda is full of Phosphic Acid (H3PO4). This stuff is a caustic polyprotic acid, with a pH roughly around the level of battery acid. It is highly water soluble, and the soda company dilutes it in bubbly water to give your drink that bite you crave without burning your throat; but the acidity has to be accounted for by your body. In order to make up for the abundance of protons flying around due to our excessive consumption of acids, the body uses alkaline compound producing minerals. To combat the Phosphoric Acid, your body has to give up some Iodine. Actually, a lot of Iodine. And the thing Iodine is essential for is thyroid function.
The thyroid hormones travel through the bloodstream to every cell in the body, where they keep everything working optimally, like oil in an engine. Without enough thyroid hormone in its active state (T3), the metabolism slows down, resulting in lowered pulse rate, coldness, weight gain, and fatigue. Other body functions slow down as well — the mind can't function normally, skin becomes dry and flaky, hair falls out, muscles become cramped, and so on.
So its not just about how many calories you consume a day, its a about consuming good healthy calories. You can eat 3,000 calories a day and gain weight, or you can eat 3,000 calories a day and lose weight.