It has been known for a while that people who switch from regular soda to diet soda (and make no other changes) do not lose weight. This is a crazy result given the calorie difference, so there must be something going on.

Somehow the body is reacting badly to the dissappoinment of not getting all the promised calories that it tasted. Either triggering cravings that drive people to make up the deficit elsewhere or causing the digestive system to change gear in some way. It is now known that there are sweetness receptors in the gut, so this flow of what the body thinks is undigested sugar may make things go haywire. Anyway, it's not my field, but I find it interesting.