I think it's because your body needs a certain amount of fat to actually function and burn calories. A lot of people doing SW report bigger losses when they have a good amount of syns and their HEx's. If you eat too little then your body will go into "starvation" mode and start to store any fat/calories you consume.
I really think if you follow the plan and have your syns you'll notice the difference. Don't be disheartened though, the beauty of SW is that you don't have to go without.
It feels weird being able to have a little of what ever you want, I've felt full plenty of times this week and feel guilty because it doesn't feel like a "normal diet" but it's not really a diet, it's teaching us to eat more normally.
Test it this week and see what happens.