Ok i try to explain it better. When we eat potatoes, our body needs to (destroy? decay? consume? Sorry i can't find correct word) make potatoes into simpler compounds and simpler and simpler and so on. To make simpler starch(potatoes), our body needs fat, if you won't provide fat in meal, it will use body fat.
Why on last meal? Because making starch simpler compounds is long time process and you have whole night for this. Also if you eat potatoes on first meal, after few hours you will eat something and you will stop this process. Also eating only carbs on first or second meal is just bad idea, you won't have energy. And eating carbs on last meal makes better sleep, and falling into sleep is easier.