Personally I think some folks just get a diet drink and decide to make up the calories (and then some) with food.
But the body does have trigger mechanisms. And it could be the case that the substances in diet sodas fool it into thinking we need to eat more.
“I think some of the artificial sweeteners actually stimulate appetite by increasing insulin output without increasing blood sugar.”—IFly4Him
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“The researchers believe that the artificial sweeteners may activate more reward centers in the brain. This in turn affects a person’s appetite, making the diet drinkers eat more food because they don’t think they are getting enough sugar in their bloodstreams.”
http://www.cbsnews.com/news/diet-soda-drinkers-consume-more-calories-study/