I think it's because they ask you to fill up on superfree, which has very little calories, so then you're likely to eat less of the other stuff than you usually do.
Also, because you have to be really strict, you need to make more stuff from scratch, and you can control what's in it.
I lived on processed food and takeaways before this week, without a thought to the calorie content. this was an eye opener and now I'm shocked at how many calories can be crammed into a sandwich! I hope it's done some good.