Lynn -- That's a great question.
I wonder if people who decide to have surgery are different. Maybe when you get to that point, and go through with it, it's just more important than anything else.
Another explanation is that, having had the surgery, people feel it's important to justify that decision -- after all, it's a huge decision to make... I know from other research that people often feel the need to reduce any dissonance they might have following such decisions. So, maybe they felt that, if it weren't so important to lose weight, why else would they have gone through all that. (Please note, I'm not questioning the decision of anyone who chooses to have WLS... I'm just saying that when we make life-altering, irreversible decisions, we feel the need to justify them).
Finally, not only were these morbidly obese people who chose to have surgery, but according to the abstract, they lost weight (we don't know how much) AND kept it off... maybe there's something about all three of those factors together...
Of course, I am just speculating here... I see the need for more research already!!!