I've been puzzling over this too - but this week I ate a LOT and lost 5.5 lbs. My take on it (and I've only been doing it for six weeks), is that it's the eating one third of super-free food at every meal that is the key to weight loss. So for the gal eating 18 muller lights a day - maybe she should have tried chopping up an apple in one muller light to follow that guideline?
I also think - and I could be wrong - but with weight watchers all food was 'equal. So I would end up eating the chocolate synthetic diet food rather than a banana, because dearie me, bananas are high in calories etc. But actually, eating loads of fruit and veg is making me feel wonderful, and it has to be easier for the body to digest and not gain weight on than synthetic diet food or chocolate. I'm not sure, but I am finding it astonishing how well it works.
Also, I'm finding it psychologically very freeing. Because I can eat as much as I want, I'm gradually eating less and less because I know I can have more at any time. That's important to me too - and fits in with what I understand is the best chance of losing weight for good, which is to make a permanent change to the way you see food rather than seeing this as a temporary fix.